Phalcon\Cache\Exception: Cached keys need to be enabled to use this function (options['statsKey'] == '_PHCM')!

My service:

$di->set('viewCache', function() use ($config) {
    $frontCache = new Phalcon\Cache\Frontend\Igbinary(array(
        "lifetime" => $config->application->viewCacheLifeTime
    ));
    $cache = new Phalcon\Cache\Backend\Memcache($frontCache, array(
        "host" => "localhost",
        "port" => "11211",
        'persistent' => false
    ));

    return $cache;
});

volt file:

{% cache "sidebar" %}
<a>generate this content is slow so we are going to cache it </a>
{% endcache %}

Controller file:

var_dump($this->viewCache->queryKeys());
die();

Error:

Phalcon\Cache\Exception: Cached keys need to be enabled to use this function (options['statsKey'] == '_PHCM')!
phalcon/cache/backend/memcache.zep (339)
on:
var_dump($this->viewCache->queryKeys());

Whats is the error?, please help!