Toplevel and syntax extension.

Dear camlers,

Consider the following script:

#use "topfind";;
#camlp4o;;
#require " sexplib.syntax";;

open Sexplib.Std;;

type t = int with sexp;;

Saved as script.ml, the simple call:

ocaml script.ml

fails while the call:

cat script.ml | ocaml

succeeds. Any idea how I could fix the first call?

Cheers,
  Philippe.