POST action fails with 404

[ghuysmans]

Hi,
I'm using Eliom 6.12.1 on OCaml 4.08.0 and the action I've defined responds with a 404, even if it actually gets executed. On the client side, I get a strange error but that's no news: that was already reported in #140. Did I do something wrong? Here's a minimal reproducible example, based on the basic.ppx template:

[%%shared
open Eliom_lib
open Eliom_content
open Html.D
]

module Badactions_app =
  Eliom_registration.App (
  struct
    let application_name = "badactions"
    let global_data_path = None
  end)

let ct = ref 0

let set_service =
  Eliom_registration.Action.create
    ~path:(Eliom_service.Path ["set"])
    ~meth:Eliom_parameter.(Eliom_service.Post (
      unit,
      int "value"))
    (fun () value ->
      Format.printf "set %d -> %d@." !ct value;
      ct := value;
      Lwt.return ())

let _ =
  Badactions_app.create
    ~path:(Eliom_service.Path [])
    ~meth:(Eliom_service.Get Eliom_parameter.unit)
    (fun () () ->
       Lwt.return
         (Eliom_tools.F.html
            ~title:"Bad Actions"
            Html.F.(body [
              h1 [txt "Bad Actions"];
              p [
                txt "current value: ";
                txt (string_of_int !ct);
              ];
              Form.post_form ~service:set_service (fun name -> [
                Form.input ~input_type:`Number ~name ~value:42 Form.int;
                button [txt "set"];
              ]) ();
            ])))