safe-string option not working in toplevel ?

  According to the section of the Manual on the String module, the String.set function should not be usable when the safe-string option is set.

  But I see no difference :

Ordinary ewandelanoy$ ocaml
        OCaml version 4.02.1


# String.set;;
Warning 3: deprecated: String.set
Use Bytes.set instead.
- : bytes -> int -> char -> unit = <fun>
# #quit;;
Ordinary ewandelanoy$ ocaml -safe-string
        OCaml version 4.02.1


# String.set;;
Warning 3: deprecated: String.set
Use Bytes.set instead.
- : bytes -> int -> char -> unit = <fun>



Am I missing something or writing something wrong ?