Activate unlocked iPhone 3GS without sim card

I recently unlocked my old iPhone 3GS through AT&T and the unlock request was approved. I have restored my iPhone to factory settings multiple times, however iTunes still says I need a SIM card to activate. From what I’ve read, I should just be able to activate it through iTunes, right? Is there any way I can activate the phone now without a SIM card or does the 3GS need one?