Retrieving value from txtbox that gets its value from a dlookup statement

I have an access form that populates a text field based on a "dlookup" statement, whenever a combo box changes.  I need to get the value in that text field when it is filled (e.g. Yes/No).  Whenever I look at the value of this field in debug mode, it is "null", so I cannot branch on its value to do something else on the form.  Not sure how to handle this.

What I would like to do is this.
1. - select a value from a combo box
2. - have this value in the field be updated with a yes / no from the database table
3. - If it's a "yes", populate another combo box with certain values, or, if it is a "no", populate the other combo box with different values.

Thanks..Tom