protected function setPaymentMethodAttr($value, $isDirty = true)
    {
        if ($value !== null && $value !== '' && !in_array($value, $this->_paymentMethods)) {
            throw new InvalidMethodException('Invalid payment method: ' . $value);
        }

        $this->paymentMethod = $value;
        $this->setDirtyState($isDirty, 'paymentMethod');
    }