Disable the rest of the checkboxes in a groupbox if I already selected two checkboxes?

 Private Sub AllBoxes_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CheckBox1.CheckedChanged, CheckBox2.CheckedChanged, CheckBox3.CheckedChanged, CheckBox4.CheckedChanged, CheckBox5.CheckedChanged, CheckBox6.CheckedChanged

    Dim qty As Int16 = 0
    Dim cb As CheckBox


    For Each cb In GroupBox8.Controls.OfType(Of CheckBox)
        If cb.Checked Then
            qty += 1
        End If
    Next

    If qty = 2 Then

        For Each cb In GroupBox8.Controls.OfType(Of CheckBox)
            If Not cb.Checked Then
                cb.Enabled = False
            End If
        Next
    Else

        For Each cb In GroupBox8.Controls.OfType(Of CheckBox)
            cb.Enabled = True
        Next
    End If
End Sub

it just doesn't work. The groupbox's name is groupbox8 and it has 6 checkboxes on it. I need to select two checkboxes.