private function createAdSection($type)
    {
        // Check Ad type
        $adTypeClassName = '\\Sokil\\Vast\\Ad\\' . $type;
        if (!class_exists($adTypeClassName)) {
            throw new \InvalidArgumentException(sprintf('Ad type %s not supported', $type));
        }
        
        // create dom node
        $adDomElement = $this->domDocument->createElement('Ad');
        $this->domDocument->documentElement->appendChild($adDomElement);

        // create type element
        $adTypeDomElement = $this->domDocument->createElement($type);
        $adDomElement->appendChild($adTypeDomElement);
        
        // create ad section
        $adSection = new $adTypeClassName($adDomElement);
        
        // cache
        $this->vastAdNodeList[] = $adSection;
        
        return $adSection;
    }