Erreur VBA lorsqu'un membre n'appartient pas à la collection

Option Explicit
Private Sub Workbook_Open()
    Set z.App = Application
End Sub

Option Explicit
Public z As New Classe1
Public WbName, WbNames As New Collection

Option Explicit
Public WithEvents App As Application
Private Sub App_WorkbookOpen(ByVal Wb As Excel.Workbook)
    If InStr(1, LCase(Wb.Name), "dummy") + _
        InStr(1, LCase(Wb.Name), "tresorerie") >= 1 Then
            WbNames.Add Item:=Wb.Name, Key:=Wb.Name
            For WbName = 1 To WbNames.Count
                MsgBox "Ouverture détectée du fichier : " & WbNames(WbName)
            Next WbName
    End If
End Sub
Private Sub App_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Range)
Dim W
If WbNames.Count > 0 Then
    For Each W In WbNames
        If Sh.Parent.Name = W Then
            MsgBox "Action"
        End If
    Next W
End If
End Sub

Option Explicit
Private Sub Workbook_Open()
    Set z.App = Application
End Sub

Option Explicit
Public z As New Classe1

Option Explicit
Public WithEvents App As Application
Private Sub App_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Range)
If Sh.Parent.Name Like "dummy*" Or Sh.Parent.Name Like "tresorerie*" Then MsgBox "Action"
End Sub