Try DSum on the field rather than the name of the control. If your control and field have the same name, then AFAIC, this is another reason why I don't allow that. You're expecting Access to default to one or the other, and the summation values are really in the field, not in one control that is replicated many times in a datasheet or continuous form. You might be OK with DSum if the field and control have the same name but I don't run into that situation as a rule.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.