diff --git a/thunar/thunar-thumbnailer.c b/thunar/thunar-thumbnailer.c
index 14c8f02c8a7b30fcb20957ca7f64d9de8e3142d7..5a10bf07ac733e0038e2507bc34ee8298665a588 100644
--- a/thunar/thunar-thumbnailer.c
+++ b/thunar/thunar-thumbnailer.c
@@ -329,8 +329,7 @@ thunar_thumbnailer_queue_async_reply (GObject      *proxy,
 
   thumbnailer = THUNAR_THUMBNAILER (job->thumbnailer);
 
-  if (THUNAR_IS_THUMBNAILER (thumbnailer) == FALSE)
-    return;
+  _thunar_return_if_fail (THUNAR_IS_THUMBNAILER (thumbnailer));
 
   _thumbnailer_lock (thumbnailer);