gtkCompletion match-selected signal callback raising bad column exception

Hello lablgtk,
I created a simple completion with a list_store containing one column of
strings. I then attached a simple match-selected callback which retrieves
the value of the row and prints it. I get this error when I try to do that:

In callback for signal match_selected, uncaught exception:
Invalid_argument("GTree.model#get: bad column")

if anyone could point out the mistake it would be great!
thanks in advance
here's the code:

open GTree
open Gobject.Data

let cols = new GTree.column_list
let col_string = cols#add string

let create_model () =
  let data = ["foo"]
  in
  let store = GTree.list_store cols in
  let fill str =
    let iter = store#append () in
    store#set ~row:iter ~column:col_string str;
  in
  List.iter fill data;
  store



let _ =
  let source_window = GWindow.window () in
  let entry = GEdit.entry ~packing:source_window#add () in
  let model = create_model () in
  let completion = GEdit.entry_completion ~model ~entry () in
    completion#set_text_column col_string;
    ignore(
      completion#connect#match_selected
    ~callback:
    (fun m i ->
       print_endline (m#get ~row:i ~column:col_string) ;
       true));
    source_window#show ();
    GMain.Main.main ()

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