Paypal Express Checkout problem with returning customers

[sealion]

I have found a problem with using Paypal Express Checkout that involves returning customers.

If a customer has not ordered from my site before then it works fine, they are asked to create an account with the information supplied by Paypal express.  Once this is done the order proceeds as intended.

But if the customer already has an account, then one they have confirmed the payment with Paypal and been returned to my site, the system tries to log them in automatically through the Joomla login system. Because no password is supplied this fails, the customer is presented with the message 'username and password do not match or you do not have an account yet'. There is no prompt to log in or anything. Instead the customer is prompted to confirm the order, when they do they just get an error message saying that they cannot place an order without a user id, there is no way to proceed and the order fails.

Has anyone come up with a solution to this? It seems to be a problem with the programming logic rather than a simple coding error.