diff --git a/src/matrix-api.c b/src/matrix-api.c
index 4b77fa6..83223cf 100644
--- a/src/matrix-api.c
+++ b/src/matrix-api.c
@@ -70,6 +70,7 @@
  * @api: A #MatrixAPI implementation
  * @content: the JSON content of the response, as a #JsonNode
  * @data: User data specified when calling original request function
+ * @err: a #GError
  *
  * A callback function to use with API calls.
  */
diff --git a/src/matrix-api.h b/src/matrix-api.h
index c0b0387..c84c192 100644
--- a/src/matrix-api.h
+++ b/src/matrix-api.h
@@ -32,7 +32,10 @@ G_BEGIN_DECLS
 typedef struct _MatrixAPIInterface MatrixAPIInterface;
 typedef struct _MatrixAPI          MatrixAPI;
 
-typedef void (*MatrixAPICallback)(MatrixAPI *api, JsonNode *content, gpointer data);
+typedef void (*MatrixAPICallback)(MatrixAPI *api,
+                                  JsonNode *content,
+                                  gpointer data,
+                                  GError **err);
 
 struct _MatrixAPIInterface {
     /*< private >*/
diff --git a/src/matrix-http-api.c b/src/matrix-http-api.c
index 49ce725..c23f556 100644
--- a/src/matrix-http-api.c
+++ b/src/matrix-http-api.c
@@ -376,7 +376,8 @@ response_callback(SoupSession *session,
                     request->callback(
                             MATRIX_API(api),
                             content,
-                            request->callback_data);
+                            request->callback_data,
+                            NULL);
                 }
             } else {
                 g_debug("Invalid response: %s", data);
diff --git a/src/test-client.c b/src/test-client.c
index 7cd0a80..cac5651 100644
--- a/src/test-client.c
+++ b/src/test-client.c
@@ -31,7 +31,7 @@ static GOptionEntry entries[] = {
 };
 
 static void
-login_finished(MatrixAPI *api, JsonNode *content, gpointer data)
+login_finished(MatrixAPI *api, JsonNode *content, gpointer data, GError **err)
 {
     JsonPath *path = json_path_new();
     JsonNode *result;