Index: /CKEditor/trunk/_source/core/dom/walker.js
===================================================================
--- /CKEditor/trunk/_source/core/dom/walker.js	(revision 6683)
+++ /CKEditor/trunk/_source/core/dom/walker.js	(revision 6684)
@@ -339,5 +339,7 @@
 	CKEDITOR.dom.element.prototype.isBlockBoundary = function( customNodeNames )
 	{
-		var nodeNameMatches = CKEDITOR.tools.extend( {}, CKEDITOR.dtd.$block, customNodeNames || {} );
+		var nodeNameMatches = customNodeNames ?
+			CKEDITOR.tools.extend( {}, CKEDITOR.dtd.$block, customNodeNames || {} ) :
+			CKEDITOR.dtd.$block;
 
 		// Don't consider floated formatting as block boundary, fall back to dtd check in that case. (#6297)