Calculation field - referencing a field with no data
I am trying to figure out a way to force a calc field to work when it references a field with no data.
Example: The calc field is called Balance Due = Amount due - Sum(all payments made)
If there are no payments yet in the system, then the field "Balance Due" remains blank. What I'd like to show is the the Amount Due if there are no payments yet. Note, I have removed the check for "do not evaluate of the field is empty". I've also tried Balance Due = Amount Due - [0+ Sum(all payments made)] which I thought would trick the system but the value comes out to 0.