apply1 missing in generated code

The compilation result of the following program is wrong (necroml version 2.10):

type s
type m<a> = | Value (s -> (a, s))

val fmap<a, b> (f : a -> b) (w : m<a>) : m<b> =
  Value<b> (\s:s ->
    let Value v = w in
    let (ws, _) = v s in
    let v' = f ws in
    (v', s)
  )

(** Module defining the specified terms *)
module MakeInterpreter (F: UNSPEC) = struct
  include F

  let ( let* ) = M.bind

  let rec fmap: 'a 'b. ('a -> 'b M.t) -> ('a m -> ('b m) M.t) M.t  =
    function f ->
    M.ret (function w ->
      M.ret (Value (function s ->
        let Value v = w in
        let* (ws, _) = apply1 v s in
        let* v' = apply1 f ws in
        M.ret (v', s))))
end

The function apply1 is used but not declared. Note that this works as expected if we remove the Value constructor and use type m<a> := s -> (a, s).