Index: /CKEditor/trunk/_source/plugins/forms/dialogs/checkbox.js
===================================================================
--- /CKEditor/trunk/_source/plugins/forms/dialogs/checkbox.js	(revision 5358)
+++ /CKEditor/trunk/_source/plugins/forms/dialogs/checkbox.js	(revision 5359)
@@ -96,5 +96,5 @@
 								{
 									// Remove attribute 'value' of checkbox #4721.
-									var checkbox = new CKEDITOR.dom.element( 'input' );
+									var checkbox = new CKEDITOR.dom.element( 'input', element.getDocument() );
 									element.copyAttributes( checkbox, { value: 1 } );
 									checkbox.replace( element );