Questions The expression contains a call of an undefined function DateAdd()

I'm getting this error message in report designer on the following expression.  DateAdd is listed as an available function.

I currently have the expression in a variable, but I was also getting the error when I embed the DateAdd funtion in a larger expression.

(note: field names are part of a third party customization)

=DateAdd([FSSOEmployee.UsrIGAHWeekDay],'h', DateDiff('h', [FSSOEmployee.UsrIGAHWeekDay], [FSSOEmployee.UsrIGAHStartTime]))

Any ideas?  Am I missing something obvious?

Thanks

Scott