This should only happen if Amount due is also empty. The "do not evaluate" checkbox only affects the situation where all fields in the expression are empty. In your case that would be when ammount due is empty and if there are no payment records in the related payment table from which you are summing payments. In cases where one field is empty and another is not, it should evaluate as though the empty field contained a zero.
Any chance that Amount due is also empty?
That would be consistent with the 0 result you get when you include the + 0 term.
That is what I thought should happen but it doesn't work that way. There is data in Amount due. I am assuming the problem is that the Sum field is referencing fields that have no data because technically the records don't exist until a payment is entered. The moment I enter data, it all works perfectly.
Yet consider this screen shot from a demo file: You can download the file at: https://dl.dropbox.com/u/78737945/SumOfRelatedRecordsTest.fp7
The calculation expression that I used was: amount + Sum ( Child::ParentID )
And I did not clear the "do not evaluate..." check box.
ah! thanks. I knew I was missing the obvious. My relationship was backwards. Sweet!