Adding and counting on a report page footer

[Natalia]

Hi all,

I'm working on a report that has to print over a
preformatted form (it's embedded as a background image)
but I have a problem with the page footer. I need to add
numbers from each page and display them in the footer but
when I try a control with =Sum([Field_Name]) I get a
#Error. Why??? Any ideas on how to make it work?

Thanks!!

Natalia