#Error showing in Sum

I'm converting an Access 97 mdb to Access 2007. There is a subform in
97 that is based on a query which has a field called Attendee that is
either 1 or 0. There is a textbox (I think, reason in next paragraph)
in the form footer of this subform that displays the sum of that
field. The control source for the textbox is =Sum([Attendee]). In
2007, I duplicated the query and the subform but cannot get the Sum
function to work. It always displays #Error. The Sum function in 2007
is in a textbox.

Now, something odd I noticed is that the name of the textbox in the 97
database is "Field219". There are 2 more textboxes on this form
called "Field220" and "Field221". The naming convention leads me to
believe that these are Access-supplied names and not user-supplied.
What kind of control uses "Field..." for the name? Am I just missing
something? Textboxes use "Text..", Labels used "Label...", but I've
never seen "Field..." to my knowledge. Is this just an Access 97

Anyway, I'm wondering if this control type is the reason I cannot get
the Sum function to work in 2007. I hope I made sense here. If not,
I'll try to explain it again. Thanks for any help or advice.