AbleCommerce Shopping Cart Software

Hi, I am doing some end stage testing with PayPal, and am able to purchase the item via a basic sandbox account in Paypal. When I click on 'Return to Merchant' it throws an exception on my website that says: "Object reference not set to an instance of an object." ... the referring page is ProcessPayPal.ashx?OrderId=15

Any ideas or suggestions?

Since this is in Sandbox mode, SSL is not enabled.

Thanks