Form with 0 payment

I currently have several forms where customers register for paid events (through PayPal). However in some instances, because of discounts offered, the amount due is sometimes zero. I was anticipating that when the amount due is zero, a PayPal transaction would not be triggered(but SmartForms would still send the confirmation email and record the registration).

This does not seem to be happening. When a customer registers for an event, but the amount due is zero, they are getting an error message, “*The donation amount should be greater than zero.” and they cannot register.

Thanks so much for your help.