Error on the final checkout page (Paypal Sandbox)

Hi !

I'm sorry to disturb you once again, but I have a problem with Marketpress during the checkout :

I'm getting an error on the final checkout page when using a PayPal sandbox account that says Missing argument 6 for MP_Gateway_Paypal_Express::smiley:oExpressCheckoutPayment()... (this repeats for arguments 3 - 6) called in plugins/marketpress/marketpress-includes/plugins-gateway/paypal-express.php

So, can you explain this problem, or help me ?