public function zoneId(array $values, $operator)
    {
        if (!in_array($operator, array('=', '!='))) {
            return $this->translation->text('Unsupported operator');
        }

        $zone = $this->zone->get(reset($values));

        if (empty($zone)) {
            return $this->translation->text('@name is unavailable', array(
                '@name' => $this->translation->text('Condition')));
        }

        return true;
    }