Index: /CKEditor/trunk/_source/plugins/undo/plugin.js
===================================================================
--- /CKEditor/trunk/_source/plugins/undo/plugin.js	(revision 5145)
+++ /CKEditor/trunk/_source/plugins/undo/plugin.js	(revision 5146)
@@ -354,5 +354,5 @@
 			// Do nothing if it was not possible to retrieve an image.
 			if ( image.contents === false )
-				return;
+				return false;
 
 			// Check if this is a duplicate. In such case, do nothing.