Why can't I Update My iPhone 3G Software?

I have a iPhone 3G. I did not have a sim card in it. I did not use the phone part of it. I just use the iPod part on it. On iTunes when I hit check for updates this message. "iTunes could not check for an update to carrier settings for your iPhone. An unknown error occurred (1631) ". What did I need to do to update my software?

It needs a sim card, even an old one.

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