Index: wp-admin/includes/nav-menu.php
===================================================================
--- wp-admin/includes/nav-menu.php	(revision 35406)
+++ wp-admin/includes/nav-menu.php	(working copy)
@@ -902,7 +902,7 @@
 		if ( class_exists( $walker_class_name, false ) )
 			$walker = new $walker_class_name;
 		else
-			return new WP_Error( 'menu_walker_not_exist', sprintf( __('The Walker class named <strong>%s</strong> does not exist.'), $walker_class_name ) );
+			return new WP_Error( 'menu_walker_not_exist', sprintf( __( 'The Walker class named %s does not exist.' ), '<strong>' . $walker_class_name . '</strong>' ) );
 
 		$some_pending_menu_items = $some_invalid_menu_items = false;
 		foreach ( (array) $menu_items as $menu_item ) {