combobox and subform problem

I have a combobox on a main form.  
The main form does not have a record source.
The record source of the combobox is a table.

I have a subform on the main form. The subform's record source
is a table (different from the one tied to the combobox).
I have a textbox in the header section of the subform.
I want to set its value to whatever the combobox's value is.
If the combobox value changes, I want the textbox value on the subform
to change.

Below is what I already have under the After Update event of the combobox.
What is going on is when a combobox selection is made, the table tied
to the subform is cleared and then populated with data related to whatever
the combobox selection is.  


Private Sub MyComboBox_AfterUpdate()

With DoCmd
   .SetWarnings False
   .Hourglass True
   .OpenQuery "DeleteTemporaryTable"
   .OpenQuery "PopulateTemporaryTable"
   .SetWarnings True
   .Hourglass False
End With
   
   Me!MySubform.Form.Requery


Adding this code to the AferUpdate event (I've tried placing it in different
sections of the code) is causing a 440 run-time error:

 Me.MySubform.Form.MyTextbox = Me.MyComboBox

What's the problem?   Thanks.