Checking Value of Checkbox and setting value for all other Checkboxes accordingly

I am trying to create something like a "Mastercheckbox" that will automatically check all other checkboxes on my worksheet. So I'm trying to start the following VBA Code as soon as I click click on the "Mastercheckbox" I am using Form Tools rather than Active X to create my checkboxes in order to make sure my VBA is compatible with any machine (I read that Active X might cause some issues) and have got the following code:

Sub Mastercheckbox()

Dim Chk As CheckBox

With ActiveSheet.Shapes("Mastercheckbox")
    If .ControlFormat.Value = False Or .ControlFormat.Value = True Then
        If .ControlFormat.Value = False Then
            For Each chk In ActiveSheet.CheckBoxes
                If Not chk.Name = "Mastercheckbox" Then
                chk.Value = False
                End If
            Next chk
        End If

        If Not .ControlFormat.Value = True Then
            For Each chk In ActiveSheet.CheckBoxes
                If Not chk.Name = "Mastercheckbox" Then
                chk.Value = True
                End If
            Next chk
        End If
     Else: MsgBox ("couldn't check value of mastercheckbox")
     End If
End With
End Sub

Everything in the For Loops works, but checking the value of the Mastercheckbox just isn't working for some reason and it jumps straight to the Else case. Can anyone help me out?