#Error on Sum

[Guest]

Good day! am getting #Error on my unbound field with a control source:
=Sum(IIf(FL_billing_ext.Form!Paid=Yes,FL_billing_ext.Form!SubTotal,0)).

The form FL_billing_ext has a record source:
SELECT billing_ext.bnum, billing_ext.bsort, billing_ext.Paid,
billing_ext.[OR number], billing_ext.Test, Test.AMOUNT, billing_ext.Qty,
billing_ext.Discount, ((100-[Discount])/100)*([AMOUNT]*[Qty]) AS SubTotal
FROM billing_ext INNER JOIN Test ON billing_ext.Test = Test.ID;

The unbound field is from the form FL_billing, its in one-to-many
relationship with FL_billing_ext.

How can I get the "Sum" and not #Error. thanks in advance!

 

[bhipwell via AccessMonster.com]

You need quotes around the word "Yes"

B