What to do when trying to put applecare on a replaced iPod?

I recently bought AppleCare and tried to register it to my iPod & a message saying

"We apologize for any inconvenience, but registration for this product requires additional documentation. Please submit your proof of purchasefor review by Apple. If your registration is accepted, your product will be covered." popped up. The problem is that the iPod I currently have was replaced by the Genius Bar a couple of months back & they did not give me a reciept. They just made me sign something electronically on their iPhones. So what can I do?