=Sum #Error

Hi,
I have 2 reports with =Sum([Amt Paid]) in the report footer.
This same, identical report exists in 2 identical databases, on the same
machine.
One report shows $0.00 if there are no entries in the Amt Paid field.
The other report shows #Error if there is no entry in the Amt Paid field.
Once I enter a value into the Amt Paid field of the erroneous report,
(via a form), it sums the values fine.
I wonder why they act differently?

Thanks,
Mark