Greying out textboxes

Hey I have a checkbox that I want a textbox linked to that turns grey if the checkbox is not clicked. I have the code under the on click event on the checkbox and this works if the box is clicked the first time..... It allows the textbox to become enabled. But I want it to if the used accidently clicks the checkbox they can click off the checkbox and the textbox goes back grey again. Im not sure if I have the code on the wrong event for the checkbox or not. Thanks.

Private Sub FlightScheduleAffected_Click()
If IsNull(Me.FlightScheduleAffected.Value) Then
Me.TimeMOCCNotified.Enabled = False
Else
Me.TimeMOCCNotified.Enabled = True
End If
End Sub