SetCurrentFolder() for FileChooserButton in Gtk# does not work

The folowing simple example code does not work in Ubuntu 13.04 (Raring):

using System;
using Gtk;

namespace Playground
{
 class MainClass
 {
  public static void Main (string[] args)
  {
   Application.Init();
   Window win = new Window(WindowType.Toplevel);
   FileChooserButton b = new FileChooserButton("bla", FileChooserAction.SelectFolder);
   b.SetCurrentFolder("/tmp");
   win.Add(b);
   win.ShowAll();
   win.Destroyed += delegate { Application.Quit(); };
   Application.Run();
  }
 }
}

The Method SetCurrentFolder() is ineffective. The same code works perfectly fine in Ubuntu <= 12.10 and Debian 6.0/7.0.