backtrace not working in ocamldebug ?

 I have a file containing just the following code :

exception Excepted;;
let f1 x=raise(Excepted);;
let f2 x=f1 (x+1);;
let f3 x=f2 (x+7);;

f3 4;;


  The file is called debug_me.ml and I compile it wth the following command :

ocamlc -g -o debug_me.out debug_me.ml 

Next, I launch the debugger with ocamldebug debug_me.out. In the debugger, I do
« run »  and « backtrace » but this last command produces no result :

    OCaml Debugger version 4.02.1

(ocd) r
Loading program... done.
Time: 16
Program end.
Uncaught exception: Debug_me.Excepted
(ocd) bt
(ocd) backtrace