What causes this type mismatch when applying a functor?

I have the following files:

SetMaker.mli

module type Element = sig  type t  val create : 'a -> t  val compare : t -> t -> int  val to_string : t -> stringendmodule type Set = sig  type t  val empty : unit -> tendmodule Make (M : Element) : Set with type t = (M.t list)

SetMaker.ml

module type Element = sig  type t  val create : 'a -> t  val compare : t -> t -> int  val to_string : t -> stringendmodule type Set = sig  type t  val empty : unit -> tendmodule Make (M:Element) = struct  type t = M.t list  let empty () = []end

main.ml

open Mylibmodule IntEl : SetMaker.Element with type t = int = struct   type t = int  let create (x:int) : t = x  let compare x y =     if x < y then -1    else if x = y then 0     else 1  let to_string = string_of_int end;;

I think some of that is possibly more verbose than it has to be but I just tried throwing around extra declarations just to see if it wouldn't fix things or perhaps give more information in the error message. In fact, in a related question (OCaml Type Mismatch Error with Functor-Based Dictionary Insertion) putting an extra type declaration solved that issue but doesn't seem to solve it here.

When I try to compile I get the error

Error: Signature mismatch:       ...       Values do not match:         val create : t -> t       is not included in         val create : 'a -> t       The type t -> t is not compatible with the type 'a -> t       Type t is not compatible with type 'a        File "lib/SetMaker.mli", line 3, characters 2-22: Expected declaration       File "bin/main.ml", line 5, characters 6-12: Actual declaration

I'm quite confused by that because I thought 'a was supposed to be a type variable which would be compatible with any type used consistently. So I would think 'a could be of type t whatever that type is.