This happened to me when I was applying for financial aid at a few colleges a couple years ago. I mailed my FAFSA to the financial aid office at the university where I was applying and about a week later it was back in my mailbox with a little note that said, "please write your ssn#". The next day I took it personally to the financial aid office and they accepted it with no problem. I'm guessing the person who received my FAFSA just didn't know my situation, since I just left that part blank. I'm guessing this is what happened to you as well. The person who received your FAFSA might be new in the FA department or just didn't know what to do with it.

If you can, I would suggest taking the FAFSA to the financial aid office yourself. Or you can always try mailing it in again, with a note explaining your situation.