Problem with icon_view#get_path_at_pos

Hello,
I'm trying to set up a context menu when I press the right mouse
button on a cell in a GTree.icon_view but I'm having trouble getting
the tree path from the x, y position of the click event. The problem
is that the program crashes when I try to access the tree_path
obtained with get_path_at_pos.
Here is the code.

test_iconview.ml

(* ocamlc -o test_iconview -I +lablgtk2 lablgtk.cma gtkInit.cmo
test_iconview.ml && ./test_iconview *)
let _ = begin
  let window = GWindow.window ~show:false () in

  let cols = new GTree.column_list in

  let col_text  = cols#add Gobject.Data.string in

  let col_pixbuf  = cols#add (Gobject.Data.gobject_by_name "GdkPixbuf") in

  let model = GTree.list_store cols in

  let sw = GBin.scrolled_window ~packing:window#add () in

  let view = GTree.icon_view ~model ~width:500 ~height:300 ~packing:sw#add () in

  view#set_text_column col_text;

  view#set_pixbuf_column col_pixbuf;
  view#set_margin 0;

  let row = model#append() in

  model#set ~row ~column:col_text "First Element";
  (* Image files from the Lablgtk examples *)

  (*model#set ~row ~column:col_pixbuf (GdkPixbuf.from_file
"gnome-fs-directory.png");*)

  let row = model#append() in

  model#set ~row ~column:col_text "Second Element";

  (*model#set ~row ~column:col_pixbuf (GdkPixbuf.from_file
"gnome-fs-regular.png");*)

(*  let tpath = view#get_path_at_pos 0 0  in
  Printf.fprintf stderr "tpath = %s;\n%!" (GTree.Path.to_string tpath);*)


  ignore (view#event#connect#button_press ~callback:begin fun ev ->
    let x, y = int_of_float (GdkEvent.Button.x ev), int_of_float
(GdkEvent.Button.y ev) in
    let tpath = view#get_path_at_pos x y in
    let row = model#get_iter tpath in
    Printf.printf "%s\n%!" (model#get ~row ~column:col_text);

    false

  end);

  ignore (window#connect#destroy ~callback:GMain.quit);
  window#show();
  GMain.main()

end

Does anyone have some idea what can be wrong?
Many thanks.

Francesco Tovagliari