Linking Thunderbolt Display to Mac Mini AppleCare

I've Recently Purchased a 2012 Mac Mini and A Thunderbolt Display. i also Bought AppleCare for the Mac Mini as well.

When i go to register the AppleCare to the Product, it gives me the option to Link a Display to it, but when i try to input the Thunderbolt Display's Serial# and go to the next step, it says that it is invalid.

What could Be the Problem? they were purchased on the same day...