Index: wp-admin/js/nav-menu.js
===================================================================
--- wp-admin/js/nav-menu.js	(revision 23954)
+++ wp-admin/js/nav-menu.js	(working copy)
@@ -528,8 +528,11 @@
 
 			processMethod = processMethod || api.addMenuItemToBottom;
 
-			if ( '' == url || 'http://' == url )
+			//Highlight the form field with red if URL is empty
+			if ( '' == url || 'http://' == url )	{
+				$('#menu-item-url-wrap').addClass('form-invalid');
 				return false;
+			}
 
 			// Show the ajax spinner
 			$('.customlinkdiv .spinner').show();
Index: wp-admin/css/wp-admin.css
===================================================================
--- wp-admin/css/wp-admin.css	(revision 23954)
+++ wp-admin/css/wp-admin.css	(working copy)
@@ -526,6 +526,7 @@
 .nav-menus-php .list-container,
 .menu-item-handle,
 .link-to-original,
+.customlinkdiv .form-invalid,
 .nav-menus-php .major-publishing-actions .form-invalid,
 .press-this #message,
 #TB_window,
@@ -7358,6 +7359,7 @@
 	text-decoration: none;
 }
 
+.customlinkdiv .form-invalid,
 .nav-menus-php .major-publishing-actions .form-invalid {
 	padding-left: 4px;
 	margin-left: -4px;