Populate a ComboBox using a popup form

[burrina]

I have a popup form that opens from another form and when the command button is clicked on the popup form, it then opens another form,and populates a combobox on that form.
Problem is this, error message: Filed cannot be updated is what I get. Of course all of the fields do get updated and the code works except for the error message. The error is being generated because I use an expression to populate the field ExpCity (on the final destination form) and it looks up the city,address,etc... Under normal conditions (on destination form) you would click on the combobox and choose a customer and it would fill in the required data, such as address,city,state,zip,phone. BUT the form is being opened via the code from the popup form and populated with that code as well. The combobox uses a query with the expression ExpCity in it that populates the city,address,zip on the destination form.

Any ideas how to get rid of the error message?