Problems with SUM

[wizcow]

I am trying to use a query as the record source for my InvoiceSub form.

I have enclosed a gif of my QBE.
If I erase the 'sum' part of the calculation, the query opens.

Anyone know what I'm doing wrong here?

Tom

ps: I enlarged the pic.

 

[neileg]

You appear to have a control on your form that has the same name as one of your fields, invoiceno and InvoiceNo. The control should have a different name.