Thanks, Rubielyn.  You didn't answer the question directly, but you did make something click in my brain.

The error message itself made no sense simply because nobody else would have a reason to be in that data base, which didn't yet exist because I was trying to restore the backup.  There are only three of us in the office today (normally just two), and I know what data bases they were working in.

The problem, as it turned out, is that I needed to be in single-user mode!  That's not stated in the error message, nor is it stated anywhere in the instructions, so it never occurred to me.  Just about anything else that needs to be done in single-user mode tells me so when I forget to switch -- another reason not to think of it.

Thanks for your help!