[AccessD] Setting Control Source of a Group to Numeric in a Report

Dear List:

I have a report which may be sorted on one of ten user defined fields which may be holding alphanumeric, numeric, or date data.  

If it is numeric I want the control source of that grouping level to be Val(Nz([fldUDF1])) 

But if I set it either through the sorting and grouping property sheet or through code it generates an error 'Extra ) in query expression '[Val(Nz([fldUDF1]))]' which is not what it means because the parentheses are balanced.

Am I trying to do something which cannot be done?

MTIA

is