diff --git a/src/wp-includes/nav-menu.php b/src/wp-includes/nav-menu.php
index 1672b03b8a..363c2ac4de 100644
--- a/src/wp-includes/nav-menu.php
+++ b/src/wp-includes/nav-menu.php
@@ -276,7 +276,7 @@ function wp_update_nav_menu_object( $menu_id = 0, $menu_data = array() ) {
  * @param int $menu_id The ID of the menu. Required. If "0", makes the menu item a draft orphan.
  * @param int $menu_item_db_id The ID of the menu item. If "0", creates a new menu item.
  * @param array $menu_item_data The menu item's data.
- * @return int The menu item's database ID or WP_Error object on failure.
+ * @return int|WP_Error The menu item's database ID or WP_Error object on failure.
  */
 function wp_update_nav_menu_item( $menu_id = 0, $menu_item_db_id = 0, $menu_item_data = array() ) {
 	$menu_id = (int) $menu_id;
@@ -284,12 +284,17 @@ function wp_update_nav_menu_item( $menu_id = 0, $menu_item_db_id = 0, $menu_item
 
 	// make sure that we don't convert non-nav_menu_item objects into nav_menu_item objects
 	if ( ! empty( $menu_item_db_id ) && ! is_nav_menu_item( $menu_item_db_id ) )
-		return new WP_Error('update_nav_menu_item_failed', __('The given object ID is not that of a menu item.'));
+		return new WP_Error( 'update_nav_menu_item_failed', __( 'The given object ID is not that of a menu item.' ) );
 
 	$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,