VS2008 / Windows XP - both with all service-packs

When the macros I write modify the source-code I put those modifications inside an UndoContext so that a single Undo will revert the source to the state it was in prior to the macro's execution.

In the following macro:
'=========
    ' Insert an 'else'-clause around the next line
    '
    Sub InsertElse1()

        Dim ep1 As EnvDTE80.EditPoint2 = CreateEditPoint()

        DTE.UndoContext.Open("")

        Try
            ep1.StartOfLine()
            ep1.Insert("else{")
            ep1.LineDown(1)
            ep1.EndOfLine()
            ep1.Insert("}")
            DTE.ExecuteCommand("Edit.FormatDocument")

        Finally

            DTE.UndoContext.Close()
        End Try
    End Sub
'============
the Edit.FormatDocument command does nothing; but when I remove the UndoContext it works as expected.

My work-around is to move the Edit.FormatDocument to after the 'End Try' - but then I need to do 2 Undo's to revert the macro's changes.