Subform lookup and totals

[Wayne311]

I have a subform within a form which is a receipt. In the subform I have a text box where a clerk can enter a number (1 thru 6) which correlates to a table (districts) with a field [district] numbers (1 thru 6). I want to display the next field in the table which has the cost [charge] for a service in that district. I don't want to add any records to this table (districts) but I do want to total multiple charges onto the receipt. I have this expression, but with the "#Name?" error.

=DLookUp("[Charge]","Districts","[District]=" & [Forms]![CaseNumber Subform1]![DistrictTextBoxEnter])

Any suggestions?

Thanks