Modules & VBA :: Cancel Parameter In Before Update Event

I am trying to use a Combobox BeforeUpdate Event to achieve a required result.

I have two combo boxes inside a frame. The first is a year number 2011, 2012, 2013, 2014

The Second is the Week No ranging from 1-52, but 1-53 in some cases and this updates accordingly with the year number selected.

When the user has selected Week 53 and then changes the year to a year where only 52 weeks exist I want to Cancel their event.

I have already made a function to determine if 53 weeks exits however when I come to do the Cancel Event, the combobox for the year doesn't change back. It stays on the users new selection.

Below is the code that I am trying to use:

Private Sub cmbWeekNoFromYear_BeforeUpdate(Cancel As Integer)
If UpdateWeekNoSelections Then
Cancel = True
Me.cmbWeekNoFromYear.Undo
End If
End Sub

'cmbWeekNoFromYear is the name of the combobox which holds the year number
'UpdateWeekNoSelections Simply returns True if I want the users change cancelled for cmbWeekNoFromYear