Problem with markers in sourceview

This test program allows markers to be set on lines in a GSourceView
widget by clicking in the left margin.  On my setup (OCaml 3.10.2-3,
Lablgtk 2.10.1-2, Ubuntu Intrepid), GTK shows critical errors after I
click a couple of times.

(bug:12461): GLib-GObject-CRITICAL **: g_object_unref: assertion
`G_IS_OBJECT (object)' failed

I've deliberately made the program turn over lots of memory; without the
added code, it takes thirty clicks or so before the same message appears.

What am I doing wrong?

-- Mike

let _ = GMain.init ()

let gensym = ref 0

let source_click (view : GSourceView.source_view) ev =
  if GdkEvent.get_type ev = `BUTTON_PRESS
      && view#get_window_type (GdkEvent.get_window ev) = `LEFT then begin
    let (x, y) =
      view#window_to_buffer_coords `LEFT
	(truncate (GdkEvent.Button.x ev))
	(truncate (GdkEvent.Button.y ev)) in
    let (iter, _) = view#get_line_at_y y in
    let buf = view#source_buffer in
    let marks =
      List.filter (fun mk -> mk#get_type = "breakpoint")
	(buf#get_markers_in_region iter iter) in
    if marks <> [] then
      buf#delete_marker (List.hd marks)
    else begin
      incr gensym;
      ignore (buf#create_marker ~name:(string_of_int !gensym)
	~typ:"breakpoint" iter);
    end
  end;

  (* Create lots of garbage *)
  let rec loop n xs = if n = 0 then () else loop (n-1) (n::xs) in
  loop 1000000 [];

  (* Swallow the event *)
  true

let main () =
  let win = GWindow.window ~width:100 ~height:100 () in
  let buf = GSourceView.source_buffer ~text:"1\n2\n3\n" () in
  let view =
    GSourceView.source_view ~source_buffer:buf
      ~show_line_markers:true
      ~packing:win#add () in
  view#set_marker_pixbuf "breakpoint"
    (GdkPixbuf.from_file "/usr/share/pixmaps/apple-red.png");
  ignore (view#event#connect#button_press (source_click view));
  win#show ();
  GMain.main ()

let _ = main ()