Index: _source/lang/en.js
===================================================================
--- _source/lang/en.js	(revision 5215)
+++ _source/lang/en.js	(working copy)
@@ -169,7 +169,8 @@
 		emailBody		: 'Message Body',
 		noAnchors		: '(No anchors available in the document)',
 		noUrl			: 'Please type the link URL',
-		noEmail			: 'Please type the e-mail address'
+		noEmail			: 'Please type the e-mail address',
+		validatePopupName	: 'Please remove white space in Popup Window Name'	
 	},
 
 	// Anchor dialog
Index: _source/plugins/link/dialogs/link.js
===================================================================
--- _source/plugins/link/dialogs/link.js	(revision 5217)
+++ _source/plugins/link/dialogs/link.js	(working copy)
@@ -91,6 +91,8 @@
 		/\s*window.open\(\s*this\.href\s*,\s*(?:'([^']*)'|null)\s*,\s*'([^']*)'\s*\)\s*;\s*return\s*false;*\s*/;
 	var popupFeaturesRegex = /(?:^|,)([^=]+)=(\d+|yes|no)/gi;
 
+	var targetFrameRegex = /\s+/g;
+	
 	var parseLink = function( editor, element )
 	{
 		var href = element ? ( element.getAttribute( '_cke_saved_href' ) || element.getAttribute( 'href' ) ) : '',
@@ -776,6 +778,14 @@
 									if ( data.target )
 										this.setValue( data.target.name );
 								},
+								validate : function( data )
+								{
+									// Popup Window Name cannot contain white space #5278.
+									var aMatch = this.getValue().match( targetFrameRegex );
+									if ( aMatch )
+										alert( editor.lang.link.validatePopupName );
+									return !aMatch;
+								},
 								commit : function( data )
 								{
 									if ( !data.target )