Access 97 expression question

What’s wrong with this report formula?

=IIf(IsNull(Adv Details![PriReg],Adv Details![Reg Name],Adv Details![PriReg])

I want the field PriReg to print if it contains something, if null, I want Reg Name to print. I originally tried this using “Renewal!” as part of the identifier, (Renewal is the query that controls the report). That gave me an error message, so I tried using “Adv Details!” because that is the table where both of the fields in question live. Now it won’t accept it in the control source field at all.

Thanks for any help. If there is a better reference out there than the few syntax help files I found in Access, I’d love to hear about it.