Index: src/wp-includes/nav-menu.php
===================================================================
--- src/wp-includes/nav-menu.php	(revision 27236)
+++ src/wp-includes/nav-menu.php	(working copy)
@@ -288,8 +288,13 @@
 
 	$menu = wp_get_nav_menu_object( $menu_id );
 
-	if ( ( ! $menu && 0 !== $menu_id ) || is_wp_error( $menu ) )
+	if ( ! $menu && 0 !== $menu_id ) {
+		return new WP_Error( 'invalid_menu_id', __( 'Invalid menu ID' ) );
+	}
+
+	if ( is_wp_error( $menu ) ) {
 		return $menu;
+	}
 
 	$defaults = array(
 		'menu-item-db-id' => $menu_item_db_id,