protected function validateClientId(PaymentTransaction $paymentTransaction, Response $response)
    {
        $paymentClientId  = $paymentTransaction->getPayment()->getClientId();
        $responseClientId = $response->getPaymentClientId();

        if (     strlen($responseClientId) > 0
            &&   $paymentClientId != $responseClientId)
        {
            throw new ValidationException("Response clientId '{$responseClientId}' does " .
                                          "not match Payment clientId '{$paymentClientId}'");
        }
    }