diff --git a/src/matrix-api-types.c b/src/matrix-api-types.c
index 78a8164..cfc936f 100644
--- a/src/matrix-api-types.c
+++ b/src/matrix-api-types.c
@@ -48,6 +48,8 @@
  * @MATRIX_API_ERROR_UNKNOWN: an error unknown to the Matrix server
  * @MATRIX_API_ERROR_UNKNOWN_TOKEN: the token provided is not known
  *                                  for the homeserver
+ * @MATRIX_API_ERROR_NOT_JSON: illegal request, the content is not
+ *                             valid JSON
  * @MATRIX_API_ERROR_UNSPECIFIED: no error code was sent by the
  *                                homeserver
  * @MATRIX_API_ERROR_UNKNOWN_ERROR: an error unknown to this library
diff --git a/src/matrix-api-types.h b/src/matrix-api-types.h
index 8322804..40de754 100644
--- a/src/matrix-api-types.h
+++ b/src/matrix-api-types.h
@@ -38,6 +38,7 @@ typedef enum {
     MATRIX_API_ERROR_FORBIDDEN,
     MATRIX_API_ERROR_UNKNOWN,
     MATRIX_API_ERROR_UNKNOWN_TOKEN,
+    MATRIX_API_ERROR_NOT_JSON,
 
     /* Allow for a lot of Matrix.org defined codes
        Do not define error codes after this! */