Lock a control

[peljo]

I want to lock a control in the subform but i cannot do it.Even though the control brancho is Null, the control cartons is not locked.Can you help me ?

Private Sub Form_BeforeUpdate(Cancel As Integer)
If IsNull(Me!branch0) Then
Me!cartons.Locked = True
End If
End Sub

 

[peljo]

Subsequently i have found out the following. it is the If IsNull that does not work , However if i write If Me!Brancho = 0 then it works.
Whys is it so ?

If Me!branch0 = 0 Then
'If IsNull(Me!branch0) Then
Me!cartons.Enabled = False
End If