Protect worksheet not workinng properly

I just tried to copy an unprotected worksheet from one workbook to another (unprotected) workbook (which has protected worksheets) and got a "Workbook is protected and cannot be changed" error message.

Why would I get this message when the Workbook is not - and never was - protected?

Even unprotecting the worksheets in the destination workbook (which shouldn't be necessary) didn't make a difference.