LablTk: CFocus.get raises an immediate exception

I’m trying to learn LablTk and I suspect I am too much of a beginner to understand why this code raises immediately an exception.
My goal is to get what widget is focused on in a LablTk window, to change it when a key is pressed inside the latter.

#require "labltk";;

open Tk;;
let window = openTk () ;;
CFocus.get ();;
(* This raises : 
# Exception: Invalid_argument "cTKtoCAMLwidget". *)