I can't see anything wrong with it, but then I don't have the luxury of knowing what the error message said.
(Always post the error message. Just because you don't understand it doesn't mean that no-one else will)
It's possible you've got a context problem. You haven't shown your XML source either. Does Table2 have a child element called sAssignedUser? If not, the call on key() won't select anything.
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference