protected function addServers($cache, $servers)
    {
        if (empty($servers)) {
            $servers = [new MemCacheServer([
                'host' => '127.0.0.1',
                'port' => 11211,
            ])];
        } else {
            foreach ($servers as $server) {
                if ($server->host === null) {
                    throw new InvalidConfigException("The 'host' property must be specified for every memcache server.");
                }
            }
        }
        if ($this->useMemcached) {
            $this->addMemcachedServers($cache, $servers);
        } else {
            $this->addMemcacheServers($cache, $servers);
        }
    }