private function makePayload()
    {
        $payload = [];
        switch ($this->type) {
            case self::TYPE_POSTBACK:
            case self::TYPE_CALL:
                $payload = ['payload' => $this->payload];
                break;
            case self::TYPE_WEB:
                $payload = ['url' => $this->payload];
                break;
            default:
                throw new UnknownTypeException;
        }

        return array_merge($payload, $this->extra);
    }