Index: nav-menu.php
===================================================================
--- nav-menu.php	(revision 21615)
+++ nav-menu.php	(working copy)
@@ -276,7 +276,10 @@
 
 	$menu = wp_get_nav_menu_object( $menu_id );
 
-	if ( ( ! $menu && 0 !== $menu_id ) || is_wp_error( $menu ) )
+	if ( !$menu && 0 !== $menu_id )
+		$menu = new WP_Error('nav_menu_id_nonexistent', __('The specified menu ID does not exist') );
+
+	if ( is_wp_error($menu) )
 		return $menu;
 
 	$menu_items = 0 == $menu_id ? array() : (array) wp_get_nav_menu_items( $menu_id, array( 'post_status' => 'publish,draft' ) );