Autopopulate Problem, i.e., Null Value

I've inherited a database (as a novice at Access) and need to modify it to
make it more efficient, i.e., it needs to autopopulate with the client's
name, address, phone number, etc., when you select the cust id. Reviewed
numerous prior posts and found this cited as a good method:
http://www.mvps.org/access/forms/frm0058.htm

Cust id is a control box. I entered the query in row source. Then I went to
the client's name control. It was previously a control box with a drop down
menu. I converted it to a text box and set the control source of the text
box. Tested the form and when cust id is selected the client's name is
autopopulated into the text box.

Problem: When I complete the form entry and try to save it, I receive the
following message: The field 'Assignment Data Sheet.Rec'd From' cannot
contain a Null value because the Required property for this field is set to
True. Enter a value in the field.

How do I resolve this Null value problem in the text box I created?
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rszebras