delimcc fragment that seems to fail on ocamlopt but work in ocamlc

gistfile1.ml

open Printf
open Delimcc

let p = new_prompt ()

let main () =
 let kr = ref (fun _ -> ()) in
 push_prompt p (fun () ->
   let foo = 15 in
   let bar = foo +
     shift0 p (fun k ->
       kr := k;
     ) in
   printf "res %d\n%!" bar;
 );
 !kr

let _ =
 let k = main () in
 k 10;
 k 15;
 let oc = open_out "foo" in
 output_delim_value oc k;
 close_out oc

let _ =
 let ic = open_in "foo" in
 let (k : int -> unit) = Marshal.from_channel ic in
 k 20;
 k 30