Hide Sheet using Workbook Open Event

I know this is simple, but I can't seem to get it to work. I want to
hide (make not visible) the HelpSheet worksheet. I was going to use
the WorkbookOpen event to make sure it is not visible each time the
workbook is opened.

Private Sub XL_WorkbookOpen(ByVal Wb As Workbook)
Worksheets("HelpSheet").Visible = xlSheetVeryHidden
End Sub

I have the above code in a class module. But the sheet is still
visible. Where did I go wrong?

Thanks,
John