Assign textbox value with DLookup

(Access 03)

I have an form with a Combo box with a list of customers. On the After-update event of the combo box I want to assign a textbox the value of the companies #1 contact. (Contacts are stored in a seperate table).

Ive tried creating a DLookup even on after-update, but I keep getting an error cant assign a value to a control without having focus.

Code:

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txtContact.text = DLookUp("[Name]","TBLContact","[COMPANYID] = Forms![FRMSalesOrder]![txtBillCustID] AND [ContactNo] = 1")

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Ive tried to add the field to the query which is the forms record source, but it jsut creates problems which is pretty complicated.

Is there an alternative??

cheers