Problems with Setting Value

[CSDunn]

Hello,
I have a subform where the 'Name' and 'Source Object' properties are
both 'frmSelectByTestSUB'. This subform resides in a main form called
'frmSelectByTest'.

In the subform, I have a combo box currently named 'Combo6' that
displays a list of student names. I need to set up functionality so
that after the name in the combo box is selected (After Update?), a
textbox that is bound to a field called 'TestGrade' will have its
value set to the content of a calculated textbox control called
'GradeText'.

I have tried this:

Private Sub Combo6_AfterUpdate()
If Not IsNull(Me.Combo6) Then
Me.TestGrade = CStr(Me.GradeText)
End If
End Sub

But when I select a value from Combo6, I get a RunTime error that
states;
"Invalid Use of Null" and the error points to Me.TestGrade =
CStr(Me.GradeText).

All I am looking to do is set the value of 'TestGrade' = to the value
of 'GradeText' after a student name has been selected from 'Combo6'.
Is the problem here that value of 'GradeText' is still Null during the
'After Update' event of 'Combo6'?

What can I do?

Thank you for your help!

CSDunn