Paypal Express Plugin: null value error into PaypalrecurringPaymentservices

Hi,

I use Paypal express plugin with NOP 3.6. When I click the confirm button, I've a null value error.
I made a debug and see that the error is into:

protected CreditCardTypeType GetPaypalCreditCardType(string creditCardType)
        {
            CreditCardTypeType creditCardTypeType = (CreditCardTypeType)Enum.Parse(typeof(CreditCardTypeType), creditCardType);
            return creditCardTypeType;
        }


creditCardType is null. Why?