Index: wp-admin/includes/ajax-actions.php
===================================================================
--- wp-admin/includes/ajax-actions.php	(revision 23200)
+++ wp-admin/includes/ajax-actions.php	(working copy)
@@ -903,6 +903,8 @@
 		);
 		echo walk_nav_menu_tree( $menu_items, 0, (object) $args );
 	}
+
+	wp_die();
 }
 
 function wp_ajax_add_meta() {