Ticket #37293: 37293.0.diff

src/wp-includes/customize/class-wp-customize-nav-menu-section.php

@@ -36 +36 @@
          */
         public function json() {
                 $exported = parent::json();
-                $exported['menu_id'] = intval( preg_replace( '/^nav_menu\[(\d+)\]/', '$1', $this->id ) );
+                $exported['menu_id'] = intval( preg_replace( '/^nav_menu\[(-?\d+)\]/', '$1', $this->id ) );
 
                 return $exported;
         }