Order creation via API fails in Magento

I am getting a rather strange error when trying to create a new order via the Magento API V1.0. Here is the basic workflow of my code:

1. Create shopping cart using the cart.create method.
2. Set a customer as 'guest' using the cart_customer.set method.
3. Add products to cart using the cart_product.add method.
4. Create the order using the cart.order method.

At this point, the cart.order method throws a SoapFault exception with the message "Call to a member function setFinalPrice() on a non-object". The same exception is thrown if I replace the cart.order method with other methods such as cart.info or cart_product.list.

Since the exception's message does not seem very descriptive, I was wondering if anybody else has ever bumped into this.

Thanks.

UPDATE: The problem is due to the fact that I am adding products to the cart using their SKUs when calling the cart_product.add method. The exception is no longer thrown when using product IDs instead of SKUs (I suspect this could be a bug). However, at this point the cart.order method returns false, so the order is still not created.