diff --git a/internal/backend.go b/internal/backend.go
index fe020f0..40b7ab2 100644
--- a/internal/backend.go
+++ b/internal/backend.go
@@ -9,12 +9,12 @@ import (
 )
 
 type Backend interface {
-	Register(username, password, device string) (user User, token string, err *models.ApiError)
-	Login(username, password, device string) (user User, token string, err *models.ApiError)
+	Register(username, password, device string) (user User, token string, err models.ApiError)
+	Login(username, password, device string) (user User, token string, err models.ApiError)
 	GetUserByToken(token string) (user User)
 	GetUserByName(userName string) User
 	GetRoomByID(id string) Room
-	Sync(token string, request sync.SyncRequest) (response *sync.SyncReply, err *models.ApiError)
+	Sync(token string, request sync.SyncRequest) (response *sync.SyncReply, err models.ApiError)
 	PublicRooms() []Room
 }
 
@@ -37,15 +37,15 @@ type User interface {
 	Name() string
 	ID() string
 	Password() string
-	CreateRoom(request createroom.Request) (Room, *models.ApiError)
-	LeaveRoom(room Room) *models.ApiError
-	SetTopic(room Room, topic string) *models.ApiError
-	SendMessage(room Room, text string) *models.ApiError
+	CreateRoom(request createroom.Request) (Room, models.ApiError)
+	LeaveRoom(room Room) models.ApiError
+	SetTopic(room Room, topic string) models.ApiError
+	SendMessage(room Room, text string) models.ApiError
 	JoinedRooms() []Room
 	ChangePassword(newPassword string)
 	Devices() []devices.Device
-	SetRoomVisibility(Room, createroom.VisibilityType) *models.ApiError
+	SetRoomVisibility(Room, createroom.VisibilityType) models.ApiError
 	Logout(token string)
 	LogoutAll()
-	JoinRoom(Room) *models.ApiError
+	JoinRoom(Room) models.ApiError
 }
diff --git a/internal/backends/memory/backend.go b/internal/backends/memory/backend.go
index d67b5e7..4c5919b 100644
--- a/internal/backends/memory/backend.go
+++ b/internal/backends/memory/backend.go
@@ -29,12 +29,12 @@ func NewBackend(hostname string) *Backend {
 		data:     make(map[string]internal.User)}
 }
 
-func (backend *Backend) Register(username, password, device string) (user internal.User, token string, err *models.ApiError) {
+func (backend *Backend) Register(username, password, device string) (user internal.User, token string, err models.ApiError) {
 	backend.mutex.Lock()
 
 	if _, ok := backend.data[username]; ok {
 		backend.mutex.Unlock()
-		return nil, "", internal.NewError(models.M_USER_IN_USE, "trying to register a user ID which has been taken")
+		return nil, "", models.NewError(models.M_USER_IN_USE, "trying to register a user ID which has been taken")
 	}
 
 	user = &User{
@@ -49,17 +49,17 @@ func (backend *Backend) Register(username, password, device string) (user intern
 	return backend.Login(username, password, device)
 }
 
-func (backend *Backend) Login(username, password, device string) (user internal.User, token string, err *models.ApiError) {
+func (backend *Backend) Login(username, password, device string) (user internal.User, token string, err models.ApiError) {
 	backend.mutex.Lock()
 	defer backend.mutex.Unlock()
 
 	user, ok := backend.data[username]
 	if !ok {
-		return nil, "", internal.NewError(models.M_FORBIDDEN, "wrong username")
+		return nil, "", models.NewError(models.M_FORBIDDEN, "wrong username")
 	}
 
 	if user.Password() != password {
-		return nil, "", internal.NewError(models.M_FORBIDDEN, "wrong password")
+		return nil, "", models.NewError(models.M_FORBIDDEN, "wrong password")
 	}
 
 	token = newToken(defaultTokenSize)
@@ -69,7 +69,7 @@ func (backend *Backend) Login(username, password, device string) (user internal.
 	return user, token, nil
 }
 
-func (backend *Backend) Sync(token string, request mSync.SyncRequest) (response *mSync.SyncReply, err *models.ApiError) {
+func (backend *Backend) Sync(token string, request mSync.SyncRequest) (response *mSync.SyncReply, err models.ApiError) {
 	backend.mutex.Lock()
 	defer backend.mutex.Unlock()
 
diff --git a/internal/backends/memory/backend_test.go b/internal/backends/memory/backend_test.go
index c24877c..5c0dbc8 100644
--- a/internal/backends/memory/backend_test.go
+++ b/internal/backends/memory/backend_test.go
@@ -18,7 +18,7 @@ func TestRegisterUser(t *testing.T) {
 	)
 
 	user, token, err := backend.Register(username, password, device)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.Equal(t, username, user.Name())
 	assert.Equal(t, password, user.Password())
 	assert.NotEmpty(t, token)
@@ -32,7 +32,7 @@ func TestRegisterUserWithAlreadyTakenName(t *testing.T) {
 	)
 
 	_, _, err := backend.Register(userName, "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	_, _, err = backend.Register(userName, "", "")
 	assert.NotNil(t, err)
@@ -47,10 +47,10 @@ func TestLogin(t *testing.T) {
 	)
 
 	_, _, err := backend.Register(userName, password, "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	_, token, err := backend.Login(userName, password, "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotZero(t, token)
 }
 
@@ -63,7 +63,7 @@ func TestLoginWithWrongCredentials(t *testing.T) {
 	)
 
 	_, _, err := backend.Register(userName, password, "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	_, _, err = backend.Login(userName, "wrong password", "")
 	assert.NotNil(t, err)
@@ -81,10 +81,10 @@ func TestLogout(t *testing.T) {
 	)
 
 	user, _, err := backend.Register(userName, password, "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	_, token, err := backend.Login(userName, password, "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotZero(t, token)
 
 	user.Logout(token)
@@ -96,7 +96,7 @@ func TestGetRoomByID(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	user, token, err := backend.Register("user", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotNil(t, user)
 	assert.NotEmpty(t, token)
 
@@ -105,7 +105,7 @@ func TestGetRoomByID(t *testing.T) {
 		Name:          "room1"}
 
 	room, err := user.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotNil(t, room)
 	assert.Equal(t, room.ID(), backend.GetRoomByID(room.ID()).ID())
 
@@ -122,7 +122,7 @@ func TestGetUserByName(t *testing.T) {
 	)
 
 	user, token, err := backend.Register(userName, "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotNil(t, user)
 	assert.NotEmpty(t, token)
 
@@ -141,7 +141,7 @@ func TestPublicRooms(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	user1, _, err := backend.Register("user1", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotNil(t, user1)
 
 	// Create first room
@@ -151,7 +151,7 @@ func TestPublicRooms(t *testing.T) {
 		Preset:        createroom.PublicChat}
 
 	room1, err := user1.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotNil(t, room1)
 
 	// Create second room
@@ -161,16 +161,16 @@ func TestPublicRooms(t *testing.T) {
 		Preset:        createroom.PublicChat}
 
 	room2, err := user1.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotNil(t, room2)
 
 	// Make room2 has 2 users
 	user2, _, err := backend.Register("user2", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotNil(t, user2)
 
 	err = user2.JoinRoom(room2)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	rooms := backend.PublicRooms()
 	assert.Len(t, rooms, 2)
diff --git a/internal/backends/memory/rooms_test.go b/internal/backends/memory/rooms_test.go
index d71529f..4ca4103 100644
--- a/internal/backends/memory/rooms_test.go
+++ b/internal/backends/memory/rooms_test.go
@@ -12,7 +12,7 @@ func TestCreateRoom(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	user, _, err := backend.Register("user1", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	request := createroom.Request{
 		RoomAliasName: "room1",
@@ -20,7 +20,7 @@ func TestCreateRoom(t *testing.T) {
 		Topic:         "topic"}
 
 	room, err := user.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.Equal(t, request.RoomAliasName, room.AliasName())
 	assert.Equal(t, request.Name, room.Name())
 	assert.Equal(t, request.Topic, room.Topic())
@@ -40,7 +40,7 @@ func TestCreateAlreadyExistingRoom(t *testing.T) {
 		Topic:         "topic"}
 
 	_, err := user.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	_, err = user.CreateRoom(request)
 	assert.NotNil(t, err)
@@ -60,7 +60,7 @@ func TestSetRoomTopic(t *testing.T) {
 
 	var newTopic = "new topic"
 	err := user.SetTopic(room, newTopic)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.Equal(t, newTopic, room.Topic())
 	assert.Equal(t, 5, len(room.Events())) // TODO: check start event count
 }
@@ -109,7 +109,7 @@ func TestRoomUserCount(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	user1, _, err := backend.Register("user1", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	request := createroom.Request{
 		RoomAliasName: "room1",
@@ -117,7 +117,7 @@ func TestRoomUserCount(t *testing.T) {
 		Topic:         "topic"}
 
 	room, err := user1.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.Len(t, room.Users(), 1)
 
 	// TODO: add join another user test
diff --git a/internal/backends/memory/user.go b/internal/backends/memory/user.go
index 221e0ad..49f5b85 100644
--- a/internal/backends/memory/user.go
+++ b/internal/backends/memory/user.go
@@ -33,10 +33,10 @@ func (user *User) Password() string {
 	return user.password
 }
 
-func (user *User) CreateRoom(request createroom.Request) (internal.Room, *models.ApiError) {
+func (user *User) CreateRoom(request createroom.Request) (internal.Room, models.ApiError) {
 	for _, existingRoom := range user.backend.rooms {
 		if existingRoom.AliasName() == request.RoomAliasName { // TODO: strip and check request room alias name before use
-			return nil, internal.NewError(models.M_ROOM_IN_USE, "")
+			return nil, models.NewError(models.M_ROOM_IN_USE, "")
 		}
 	}
 
@@ -104,12 +104,12 @@ func (user *User) CreateRoom(request createroom.Request) (internal.Room, *models
 	return room, nil
 }
 
-func (user *User) SetTopic(room internal.Room, topic string) *models.ApiError {
+func (user *User) SetTopic(room internal.Room, topic string) models.ApiError {
 	room.(*Room).mutex.Lock()
 	defer room.(*Room).mutex.Unlock()
 
 	if room.(*Room).creator.ID() != user.ID() { // TODO: currently only creator can change topic
-		return internal.NewError(models.M_FORBIDDEN, "")
+		return models.NewError(models.M_FORBIDDEN, "")
 	}
 
 	room.(*Room).topic = topic
@@ -122,7 +122,7 @@ func (user *User) SetTopic(room internal.Room, topic string) *models.ApiError {
 	return nil
 }
 
-func (user *User) LeaveRoom(room internal.Room) *models.ApiError {
+func (user *User) LeaveRoom(room internal.Room) models.ApiError {
 	room.(*Room).mutex.Lock()
 	defer room.(*Room).mutex.Unlock()
 
@@ -133,10 +133,10 @@ func (user *User) LeaveRoom(room internal.Room) *models.ApiError {
 		}
 	}
 
-	return internal.NewError(models.M_BAD_STATE, "you are not a member of group") // TODO: check error code
+	return models.NewError(models.M_BAD_STATE, "you are not a member of group") // TODO: check error code
 }
 
-func (user *User) SendMessage(room internal.Room, text string) *models.ApiError {
+func (user *User) SendMessage(room internal.Room, text string) models.ApiError {
 	room.(*Room).mutex.Lock()
 	defer room.(*Room).mutex.Unlock()
 
@@ -148,7 +148,7 @@ func (user *User) SendMessage(room internal.Room, text string) *models.ApiError
 	}
 
 	if !userInRoom {
-		return internal.NewError(models.M_FORBIDDEN, "")
+		return models.NewError(models.M_FORBIDDEN, "")
 	}
 
 	room.(*Room).events = append(room.(*Room).events, RoomEvent{
@@ -195,9 +195,9 @@ func (user *User) Devices() []devices.Device {
 	return result
 }
 
-func (user *User) SetRoomVisibility(room internal.Room, visibilityType createroom.VisibilityType) *models.ApiError {
+func (user *User) SetRoomVisibility(room internal.Room, visibilityType createroom.VisibilityType) models.ApiError {
 	if user.ID() != room.Creator().ID() {
-		return internal.NewError(models.M_FORBIDDEN, "only room owner can change visibility") // TODO: room administrators can use this method too
+		return models.NewError(models.M_FORBIDDEN, "only room owner can change visibility") // TODO: room administrators can use this method too
 	}
 
 	room.(*Room).mutex.Lock()
@@ -223,7 +223,7 @@ func (user *User) LogoutAll() {
 	user.Tokens = make(map[string]Token)
 }
 
-func (user *User) JoinRoom(room internal.Room) *models.ApiError {
+func (user *User) JoinRoom(room internal.Room) models.ApiError {
 	memRoom := room.(*Room)
 
 	memRoom.mutex.Lock()
@@ -231,7 +231,7 @@ func (user *User) JoinRoom(room internal.Room) *models.ApiError {
 
 	for _, roomUser := range memRoom.joined {
 		if roomUser.ID() == user.ID() {
-			return internal.NewError(models.M_BAD_STATE, "user already in room") // TODO: check code
+			return models.NewError(models.M_BAD_STATE, "user already in room") // TODO: check code
 		}
 	}
 
diff --git a/internal/backends/memory/user_test.go b/internal/backends/memory/user_test.go
index 53e93cf..1cc06cc 100644
--- a/internal/backends/memory/user_test.go
+++ b/internal/backends/memory/user_test.go
@@ -19,7 +19,7 @@ func TestUserID(t *testing.T) {
 
 	backend := NewBackend(hostName)
 	user, _, err := backend.Register(userName, "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	assert.Equal(t, expectedUserID, user.ID())
 }
@@ -28,34 +28,34 @@ func TestUserMessage(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	user, _, err := backend.Register("user1", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	request := createroom.Request{
 		RoomAliasName: "room1",
 		Name:          "room1"}
 
 	room, err := user.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	err = user.SendMessage(room, "hello")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 }
 
 func TestUserMessageInWrongRoom(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	user1, _, err := backend.Register("user1", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	request := createroom.Request{
 		RoomAliasName: "room1",
 		Name:          "room1"}
 
 	room, err := user1.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	user2, _, err := backend.Register("user2", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	err = user2.SendMessage(room, "hello")
 	assert.NotNil(t, err)
@@ -65,7 +65,7 @@ func TestGetUserByToken(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	user, token, err := backend.Register("user1", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotEmpty(t, token)
 
 	gotUser := backend.GetUserByToken(token)
@@ -76,7 +76,7 @@ func TestGetUserByWrongToken(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	_, token, err := backend.Register("user1", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotEmpty(t, token)
 
 	gotUser := backend.GetUserByToken("wrong token")
@@ -92,10 +92,10 @@ func TestLogoutWithWrongToken(t *testing.T) {
 	)
 
 	user, _, err := backend.Register(userName, password, "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	_, token, err := backend.Login(userName, password, "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotZero(t, token)
 
 	user.Logout("worng token")
@@ -105,7 +105,7 @@ func TestJoinedRooms(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	user, _, err := backend.Register("user1", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	request := createroom.Request{
 		RoomAliasName: "room1",
@@ -113,7 +113,7 @@ func TestJoinedRooms(t *testing.T) {
 		Topic:         "topic"}
 
 	room, err := user.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	rooms := user.JoinedRooms()
 	assert.Equal(t, []internal.Room{room}, rooms)
@@ -125,7 +125,7 @@ func TestNewPassword(t *testing.T) {
 	var newPassword = "new password"
 
 	user, _, err := backend.Register("user1", "old password", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	user.ChangePassword(newPassword)
 	assert.Equal(t, newPassword, user.Password())
@@ -137,7 +137,7 @@ func TestDevices(t *testing.T) {
 	var expectedDeviceID = "my device"
 
 	user, _, err := backend.Register("user1", "", expectedDeviceID)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	devices := user.Devices()
 	assert.Len(t, devices, 1)
@@ -148,7 +148,7 @@ func TestSetRoomVisibility(t *testing.T) {
 	backend := NewBackend("localhost")
 
 	user, _, err := backend.Register("user1", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	request := createroom.Request{
 		RoomAliasName: "room1",
@@ -156,17 +156,17 @@ func TestSetRoomVisibility(t *testing.T) {
 		Visibility:    createroom.VisibilityTypePrivate}
 
 	room, err := user.CreateRoom(request)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.NotNil(t, room)
 	assert.Equal(t, createroom.VisibilityTypePrivate, room.Visibility())
 
 	err = user.SetRoomVisibility(room, createroom.VisibilityTypePublic)
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.Equal(t, createroom.VisibilityTypePublic, room.Visibility())
 
 	// TODO: Only owner can change room visibility
 	notOwnerUser, _, err := backend.Register("user2", "", "")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 
 	err = notOwnerUser.SetRoomVisibility(room, createroom.VisibilityTypePrivate)
 	assert.NotNil(t, err)
@@ -182,11 +182,11 @@ func TestLogoutAll(t *testing.T) {
 	)
 
 	user, _, err := backend.Register(userName, password, "dev1")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.Len(t, user.Devices(), 1)
 
 	_, _, err = backend.Login(userName, password, "dev2")
-	assert.Nil(t, err)
+	assert.NoError(t, err)
 	assert.Len(t, user.Devices(), 2)
 
 	user.LogoutAll()
diff --git a/internal/errors.go b/internal/errors.go
index 0bfcf75..99c6ece 100644
--- a/internal/errors.go
+++ b/internal/errors.go
@@ -6,21 +6,14 @@ import (
 	"github.com/nxshock/signaller/internal/models"
 )
 
-func errorResponse(w http.ResponseWriter, code models.ApiError, httpCode int, message string) {
+func errorResponse(w http.ResponseWriter, code models.ApiError, httpCode int, messageOverride string) {
 	w.Header().Set("Content-Type", "application/json")
 
-	if message != "" {
-		code.Message = message
-	}
-
 	w.WriteHeader(httpCode)
-	w.Write(code.JSON())
-}
 
-func NewError(code models.ApiError, message string) *models.ApiError {
-	if message != "" {
-		code.Message = message
+	if messageOverride != "" {
+		w.Write(models.NewError(code, messageOverride).JSON())
+	} else {
+		w.Write(code.JSON())
 	}
-
-	return &code
 }
diff --git a/internal/handlers.go b/internal/handlers.go
index 4ba8c58..f458c95 100644
--- a/internal/handlers.go
+++ b/internal/handlers.go
@@ -68,7 +68,7 @@ func LoginHandler(w http.ResponseWriter, r *http.Request) {
 
 			_, token, apiErr := currServer.Backend.Login(request.Identifier.User, request.Password, request.DeviceID)
 			if apiErr != nil {
-				errorResponse(w, *apiErr, http.StatusForbidden, "")
+				errorResponse(w, apiErr, http.StatusForbidden, "")
 				return
 			}
 
@@ -110,7 +110,7 @@ func leaveRoomHandler(w http.ResponseWriter, r *http.Request) {
 
 	err := user.LeaveRoom(room)
 	if err != nil {
-		errorResponse(w, *err, http.StatusBadRequest, "")
+		errorResponse(w, err, http.StatusBadRequest, "")
 		return
 	}
 
@@ -185,7 +185,7 @@ func RegisterHandler(w http.ResponseWriter, r *http.Request) {
 
 	_, token, apiErr := currServer.Backend.Register(request.Username, request.Password, request.DeviceID)
 	if apiErr != nil {
-		errorResponse(w, *apiErr, http.StatusBadRequest, "")
+		errorResponse(w, apiErr, http.StatusBadRequest, "")
 		return
 	}
 
diff --git a/internal/models/errors.go b/internal/models/errors.go
index 3f8f614..e14ee70 100644
--- a/internal/models/errors.go
+++ b/internal/models/errors.go
@@ -4,49 +4,86 @@ import (
 	"encoding/json"
 )
 
-type ApiError struct {
-	Code    string `json:"errcode"`
-	Message string `json:"error"`
+type ApiError interface {
+	error
+	Code() string
+	Message() string
+	JSON() []byte
+}
+
+type apiError struct {
+	code    string `json:"errcode"`
+	message string `json:"error"`
+}
+
+func (apiError *apiError) Error() string {
+	s := apiError.code
+
+	if apiError.message != "" {
+		s = s + ": " + apiError.message
+	}
+
+	return s
+}
+
+func (apiError *apiError) Code() string {
+	return apiError.code
+}
+
+func (apiError *apiError) Message() string {
+	return apiError.message
+}
+
+func (apiError *apiError) JSON() []byte {
+	b, _ := json.Marshal(apiError) // TODO: error handler?
+	return b
 }
 
 var (
 	// https://matrix.org/docs/spec/client_server/latest#api-standards
 
-	M_FORBIDDEN      = ApiError{"M_FORBIDDEN", ""}      // Forbidden access, e.g. joining a room without permission, failed login.
-	M_UNKNOWN_TOKEN  = ApiError{"M_UNKNOWN_TOKEN", ""}  // The access token specified was not recognised.
-	M_MISSING_TOKEN  = ApiError{"M_MISSING_TOKEN", ""}  // No access token was specified for the request.
-	M_BAD_JSON       = ApiError{"M_BAD_JSON", ""}       // Request contained valid JSON, but it was malformed in some way, e.g. missing required keys, invalid values for keys.
-	M_NOT_JSON       = ApiError{"M_NOT_JSON", ""}       // Request did not contain valid JSON.
-	M_NOT_FOUND      = ApiError{"M_NOT_FOUND", ""}      // No resource was found for this request.
-	M_LIMIT_EXCEEDED = ApiError{"M_LIMIT_EXCEEDED", ""} // Too many requests have been sent in a short period of time. Wait a while then try again.
-	M_UNKNOWN        = ApiError{"M_UNKNOWN", ""}        // An unknown error has occurred.
+	M_FORBIDDEN      = &apiError{"M_FORBIDDEN", ""}      // Forbidden access, e.g. joining a room without permission, failed login.
+	M_UNKNOWN_TOKEN  = &apiError{"M_UNKNOWN_TOKEN", ""}  // The access token specified was not recognised.
+	M_MISSING_TOKEN  = &apiError{"M_MISSING_TOKEN", ""}  // No access token was specified for the request.
+	M_BAD_JSON       = &apiError{"M_BAD_JSON", ""}       // Request contained valid JSON, but it was malformed in some way, e.g. missing required keys, invalid values for keys.
+	M_NOT_JSON       = &apiError{"M_NOT_JSON", ""}       // Request did not contain valid JSON.
+	M_NOT_FOUND      = &apiError{"M_NOT_FOUND", ""}      // No resource was found for this request.
+	M_LIMIT_EXCEEDED = &apiError{"M_LIMIT_EXCEEDED", ""} // Too many requests have been sent in a short period of time. Wait a while then try again.
+	M_UNKNOWN        = &apiError{"M_UNKNOWN", ""}        // An unknown error has occurred.
 
-	M_UNRECOGNIZED                    = ApiError{"M_UNRECOGNIZED", ""}                    // The server did not understand the request.
-	M_UNAUTHORIZED                    = ApiError{"M_UNAUTHORIZED", ""}                    // The request was not correctly authorized. Usually due to login failures.
-	M_USER_IN_USE                     = ApiError{"M_USER_IN_USE", ""}                     // Encountered when trying to register a user ID which has been taken.
-	M_INVALID_USERNAME                = ApiError{"M_INVALID_USERNAME", ""}                // Encountered when trying to register a user ID which is not valid.
-	M_ROOM_IN_USE                     = ApiError{"M_ROOM_IN_USE", ""}                     // Sent when the room alias given to the createRoom API is already in use.
-	M_INVALID_ROOM_STATE              = ApiError{"M_INVALID_ROOM_STATE", ""}              // Sent when the initial state given to the createRoom API is invalid.
-	M_THREEPID_IN_USE                 = ApiError{"M_THREEPID_IN_USE", ""}                 // Sent when a threepid given to an API cannot be used because the same threepid is already in use.
-	M_THREEPID_NOT_FOUND              = ApiError{"M_THREEPID_NOT_FOUND", ""}              // Sent when a threepid given to an API cannot be used because no record matching the threepid was found.
-	M_THREEPID_AUTH_FAILED            = ApiError{"M_THREEPID_AUTH_FAILED", ""}            // Authentication could not be performed on the third party identifier.
-	M_THREEPID_DENIED                 = ApiError{"M_THREEPID_DENIED", ""}                 // The server does not permit this third party identifier. This may happen if the server only permits, for example, email addresses from a particular domain.
-	M_SERVER_NOT_TRUSTED              = ApiError{"M_SERVER_NOT_TRUSTED", ""}              // The client's request used a third party server, eg. identity server, that this server does not trust.
-	M_UNSUPPORTED_ROOM_VERSION        = ApiError{"M_UNSUPPORTED_ROOM_VERSION", ""}        // The client's request to create a room used a room version that the server does not support.
-	M_INCOMPATIBLE_ROOM_VERSION       = ApiError{"M_INCOMPATIBLE_ROOM_VERSION", ""}       // The client attempted to join a room that has a version the server does not support. Inspect the room_version property of the error response for the room's version.
-	M_BAD_STATE                       = ApiError{"M_BAD_STATE", ""}                       // The state change requested cannot be performed, such as attempting to unban a user who is not banned.
-	M_GUEST_ACCESS_FORBIDDEN          = ApiError{"M_GUEST_ACCESS_FORBIDDEN", ""}          // The room or resource does not permit guests to access it.
-	M_CAPTCHA_NEEDED                  = ApiError{"M_CAPTCHA_NEEDED", ""}                  // A Captcha is required to complete the request.
-	M_CAPTCHA_INVALID                 = ApiError{"M_CAPTCHA_INVALID", ""}                 // The Captcha provided did not match what was expected.
-	M_MISSING_PARAM                   = ApiError{"M_MISSING_PARAM", ""}                   // A required parameter was missing from the request.
-	M_INVALID_PARAM                   = ApiError{"M_INVALID_PARAM", ""}                   // A parameter that was specified has the wrong value. For example, the server expected an integer and instead received a string.
-	M_TOO_LARGE                       = ApiError{"M_TOO_LARGE", ""}                       // The request or entity was too large.
-	M_EXCLUSIVE                       = ApiError{"M_EXCLUSIVE", ""}                       // The resource being requested is reserved by an application service, or the application service making the request has not created the resource.
-	M_RESOURCE_LIMIT_EXCEEDED         = ApiError{"M_RESOURCE_LIMIT_EXCEEDED", ""}         // The request cannot be completed because the homeserver has reached a resource limit imposed on it. For example, a homeserver held in a shared hosting environment may reach a resource limit if it starts using too much memory or disk space. The error MUST have an admin_contact field to provide the user receiving the error a place to reach out to. Typically, this error will appear on routes which attempt to modify state (eg: sending messages, account data, etc) and not routes which only read state (eg: /sync, get account data, etc).
-	M_CANNOT_LEAVE_SERVER_NOTICE_ROOM = ApiError{"M_CANNOT_LEAVE_SERVER_NOTICE_ROOM", ""} // The user is unable to reject an invite to join the server notices room. See the Server Notices module for more information.
+	M_UNRECOGNIZED                    = &apiError{"M_UNRECOGNIZED", ""}                    // The server did not understand the request.
+	M_UNAUTHORIZED                    = &apiError{"M_UNAUTHORIZED", ""}                    // The request was not correctly authorized. Usually due to login failures.
+	M_USER_IN_USE                     = &apiError{"M_USER_IN_USE", ""}                     // Encountered when trying to register a user ID which has been taken.
+	M_INVALID_USERNAME                = &apiError{"M_INVALID_USERNAME", ""}                // Encountered when trying to register a user ID which is not valid.
+	M_ROOM_IN_USE                     = &apiError{"M_ROOM_IN_USE", ""}                     // Sent when the room alias given to the createRoom API is already in use.
+	M_INVALID_ROOM_STATE              = &apiError{"M_INVALID_ROOM_STATE", ""}              // Sent when the initial state given to the createRoom API is invalid.
+	M_THREEPID_IN_USE                 = &apiError{"M_THREEPID_IN_USE", ""}                 // Sent when a threepid given to an API cannot be used because the same threepid is already in use.
+	M_THREEPID_NOT_FOUND              = &apiError{"M_THREEPID_NOT_FOUND", ""}              // Sent when a threepid given to an API cannot be used because no record matching the threepid was found.
+	M_THREEPID_AUTH_FAILED            = &apiError{"M_THREEPID_AUTH_FAILED", ""}            // Authentication could not be performed on the third party identifier.
+	M_THREEPID_DENIED                 = &apiError{"M_THREEPID_DENIED", ""}                 // The server does not permit this third party identifier. This may happen if the server only permits, for example, email addresses from a particular domain.
+	M_SERVER_NOT_TRUSTED              = &apiError{"M_SERVER_NOT_TRUSTED", ""}              // The client's request used a third party server, eg. identity server, that this server does not trust.
+	M_UNSUPPORTED_ROOM_VERSION        = &apiError{"M_UNSUPPORTED_ROOM_VERSION", ""}        // The client's request to create a room used a room version that the server does not support.
+	M_INCOMPATIBLE_ROOM_VERSION       = &apiError{"M_INCOMPATIBLE_ROOM_VERSION", ""}       // The client attempted to join a room that has a version the server does not support. Inspect the room_version property of the error response for the room's version.
+	M_BAD_STATE                       = &apiError{"M_BAD_STATE", ""}                       // The state change requested cannot be performed, such as attempting to unban a user who is not banned.
+	M_GUEST_ACCESS_FORBIDDEN          = &apiError{"M_GUEST_ACCESS_FORBIDDEN", ""}          // The room or resource does not permit guests to access it.
+	M_CAPTCHA_NEEDED                  = &apiError{"M_CAPTCHA_NEEDED", ""}                  // A Captcha is required to complete the request.
+	M_CAPTCHA_INVALID                 = &apiError{"M_CAPTCHA_INVALID", ""}                 // The Captcha provided did not match what was expected.
+	M_MISSING_PARAM                   = &apiError{"M_MISSING_PARAM", ""}                   // A required parameter was missing from the request.
+	M_INVALID_PARAM                   = &apiError{"M_INVALID_PARAM", ""}                   // A parameter that was specified has the wrong value. For example, the server expected an integer and instead received a string.
+	M_TOO_LARGE                       = &apiError{"M_TOO_LARGE", ""}                       // The request or entity was too large.
+	M_EXCLUSIVE                       = &apiError{"M_EXCLUSIVE", ""}                       // The resource being requested is reserved by an application service, or the application service making the request has not created the resource.
+	M_RESOURCE_LIMIT_EXCEEDED         = &apiError{"M_RESOURCE_LIMIT_EXCEEDED", ""}         // The request cannot be completed because the homeserver has reached a resource limit imposed on it. For example, a homeserver held in a shared hosting environment may reach a resource limit if it starts using too much memory or disk space. The error MUST have an admin_contact field to provide the user receiving the error a place to reach out to. Typically, this error will appear on routes which attempt to modify state (eg: sending messages, account data, etc) and not routes which only read state (eg: /sync, get account data, etc).
+	M_CANNOT_LEAVE_SERVER_NOTICE_ROOM = &apiError{"M_CANNOT_LEAVE_SERVER_NOTICE_ROOM", ""} // The user is unable to reject an invite to join the server notices room. See the Server Notices module for more information.
 )
 
-func (apiError ApiError) JSON() []byte {
-	b, _ := json.Marshal(apiError) // TODO: error handler?
-	return b
+func NewError(err ApiError, messageOverride string) ApiError {
+	newErr := &apiError{
+		code:    err.Code(),
+		message: err.Message()}
+
+	if messageOverride != "" {
+		newErr.message = messageOverride
+	}
+
+	return newErr
 }