diff --git a/lib/thumbnail.lib.php b/lib/thumbnail.lib.php
index 3609eb285..7f95f82b5 100644
--- a/lib/thumbnail.lib.php
+++ b/lib/thumbnail.lib.php
@@ -213,6 +213,8 @@ function thumbnail($filename, $source_path, $target_path, $thumb_width, $thumb_h
         $src = imagecreatefromjpeg($source_file);
     } else if ($size[2] == 3) {
         $src = imagecreatefrompng($source_file);
+    } else {
+        return;
     }
 
     if(!$src)