I bought an unlocked iPhone 5 in the US. It's not working in Japan. Is there a limit I'm not aware of?

I bought an unlocked iPhone 5 in the US. Reluctantly Japan's Softbank agreed to activate it with a SIM card for me. But it didn't work, and instead I got this error message:

SIM Not Valid

The SIM card that you currently have in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported carrier or request that this iPhone be unlocked by your carrier. Please contact Apple for more information.

I bought this phone on eBay, it was listed as unlocked. Is this an international thing? Thanks to any and all that can help.