IVsOutputWindowPane.Activate E_UNEXPECTED

Any ideas what could cause IVsOutputWindowPane.Activate to fail with E_UNEXPECTED error code.

In my case the code is something like this:

Guid buildPaneGuid = VSConstants.OutputWindowPaneGuid.BuildOutputPane_guid;
IVsOutputWindowPane pane;
outputWindow.GetPane(ref buildPaneGuid, out pane);
OutputPane = pane;
...
//
// Here I dispatch the action to the UI thread
//
Dispatcher.BeginInvoke(DispatcherPriority.Send,
    new Action(() =>
    {
        OutputPane.Activate(); // E_UNEXPECTED
        ....
    }));