Unsocial security number

[pabba]

Here's a question I just asked on my Twitter, but I hope some of my writerly LJ friends can help me with a solution:

I'm signing a short story contract at the moment, and they are asking for my SSN/SIN/TAX ID. Can I opt out of this?

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Has anyone else done this? I just feel weird about giving out my SSN, and I don't even know what my SIN and TAX ID are...