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Just saw this question now and that it has no answer yet. If you haven't solved then there is a couple of things you can try.
One thing I never do is keep the control names created by a form or report wizard. Rather, I use a standard naming convention, prefacing the field name by type:
- not Lname, txtLname, txtCurDate not CurDate
The reason is to remove any possible ambiguity between a control and a field name, especially when it comes to aggregate functions. You can end up summing a field when you want to sum what a control contains.
The other possible fix might be domain aggregate functions such as DSum and not Sum, but those are for summing table values (usually over groups). If these ideas don't help, consider posting a copy of your db which contains just enough to replicate the issue (compacted and zipped) for analysis.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.