Combo box on a form

Hi I have a combo box on a form named "shift" I populate the field by a value list. I want to stop the user from leaving the field blank. I have tried using the validate property but cant get it to work. I have tried code

Private Sub Shift_BeforeUpdate(Cancel As Integer)

If Me.Shift = "" Or IsNull Then
MsgBox "You Must enter a valid Shift here"
Cancel = True
End If

End Sub

This does not work either.

Not sure what to try next any help would be great

Thanks Bob