Overflow Runtime Error 6

Hello,

When I try to select all on a sheet by clicking the upper left box I get this error. Does anyone know why and how to fix please?

Thank you

debug at:

If (Sh.Index < 4) + (Target.Count > 1) Then Exit Sub

Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Range)
    If (Sh.Index < 4) + (Target.Count > 1) Then Exit Sub
    If Target.Column = 15 Then
        If Len(Trim(Target.Value)) = 0 Then
            Target.Value = "X"
        Else
            Target.ClearContents
        End If
    End If
End Sub