Sum in form doesn't work

[rperre]

Hey, I'm trying to calculate a Sum of Payment amounts in a form. However I get a #error no matter what I try.

The form uses a table as source and there are no filters running. I have another similar form and it works on that one, I'm puzzled??????????

This is the source code I use:

=abs(sum(pmt(rate,nper,amount)))

If I calculate the pmt in the detail section, it works as advertised, but doing it in the footer with the sum command it gives me a #error

what's wrong with this, or with the form

thanks,

Richard