Cheak boxes .value help

[ichar]

basically i've got two chk boxes. one called chkExtension and one called chkSpecialEvent.

When chkSpecailEvent is turned on (i.e. the tick is put in thier) both chkExtension and chkSpecialEvent must be checked.

when When chkSpecailEvent is turned off(i.e. the tick is taken out) both chkExtension and chkSpecialEvent must not be checked.

When chkExtension is turned on (i.e. the tick is put in thier) chkExtension must stayon, while chkSpecailEvent is turned off.

When chkExtension is turned okk(i.e. the tick is taken out) chkExtension must stayon, while chkSpecailEvent is turned off.

Okay i hope thats clear. now i'm using this code

Private Sub chkExtension_Click()
    If chkExtension.Value = True Then
        chkSpecialEvent.Value = False
        chkExtension.Value = False
    ElseIf chkExtension.Value = False Then
        chkSpecialEvent.Value = False
        chkExtension.Value = True
    End If
End Sub
 
Private Sub chkSpecialEvent_Click()
    If chkSpecialEvent.Value = 1 Then
        chkSpecialEvent.Value = 0
        chkExtension.Value = 0
    ElseIf chkSpecialEvent.Value = 0 Then
        chkSpecialEvent.Value = 1
        chkExtension.Value = 1
    End If
    
End Sub

but when i run it, it says out of stack? not a clue what that means. could anyone help me?