diff --git a/builder/builder-utils.c b/builder/builder-utils.c
index 97445aab9..6ccf9f5ff 100644
--- a/builder/builder-utils.c
+++ b/builder/builder-utils.c
@@ -1700,7 +1700,8 @@ builder_serializable_find_property_with_error (JsonSerializable *serializable,
                                                const char       *name)
 {
   GParamSpec *pspec = g_object_class_find_property (G_OBJECT_GET_CLASS (serializable), name);
-  if (pspec == NULL)
+  if (pspec == NULL &&
+      !g_str_has_prefix (name, "x-"))
     g_warning ("Unknown property %s for type %s\n", name, g_type_name_from_instance ((GTypeInstance *)serializable));
   return pspec;
 }