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Here is an example database I've created. If you open the form and use the combo box in field C you will see the issue in the status bar.

I cannot post the db because I am using SQL as a backend. Can are you able to recreate my error based on the earlier reply?

It seems that none of the events fire until the error is given. If you make any calculated field a combo box whether the field is calculated within the field itself or in a query as soon as you select a value from the dropdown of the combo box it gives the follwowing error in the status bar...

Example I have 2 fields in a query: A = [null] B = "Test" In a form I have a field (C) where the control source is IIf(IsNull(A),B,A) What I'd like to do is when a new value is selected in the combo box it writes that value to the A field and then the field recalculates The form field...