How to set the Credit Card Select to null using PHP Toolkit?

When creating a new Sales Order is defaulting the credit card to the one set as default in the customer record. We can set the Payment Method to PayPal and enter the PP transaction details, but it still somehow grabbing the card. How can we override that?

We've tried the following:
$salesOrder->creditCard = new RecordRef();
$salesOrder->creditCard.internalId = new NullField();

We've also tried:
$salesOrder->creditCard = new NullField();
$salesOrder->creditCard.internalId = new NullField();

Neither of which worked. Are we missing something here?

Thanks!