protected function registerMuLoaderLoader()
    {
        // Add mu loader if set
        if (defined('WPMU_LOADER')) {
            $this->addFilter('muplugins_loaded', function () {
                if (defined('WPMU_LOADER') && file_exists(WPMU_PLUGIN_DIR . DIRECTORY_SEPARATOR . WPMU_LOADER)) {
                    require_once(WPMU_PLUGIN_DIR . DIRECTORY_SEPARATOR . WPMU_LOADER);
                }
            }, PHP_INT_MIN);
        }
    }