diff --git a/src/module.ml b/src/module.ml
index 70d930e9c26..aa2c8bef48c 100644
--- a/src/module.ml
+++ b/src/module.ml
@@ -269,25 +269,25 @@ let obj_name t = t.obj_name
 
 let cm_source t kind = file t (Cm_kind.source kind)
 
-let cm_file_unsafe t ?ext kind =
-  let ext = Option.value ext ~default:(Cm_kind.ext kind) in
+let cm_file_unsafe t kind =
+  let ext = Cm_kind.ext kind in
   obj_file t ~kind ~ext
 
-let cm_file t ?ext (kind : Cm_kind.t) =
+let cm_file t (kind : Cm_kind.t) =
   match kind with
   | (Cmx | Cmo) when not (has_impl t) -> None
-  | _ -> Some (cm_file_unsafe t ?ext kind)
+  | _ -> Some (cm_file_unsafe t kind)
 
-let cm_public_file_unsafe t ?ext kind =
-  let ext = Option.value ext ~default:(Cm_kind.ext kind) in
+let cm_public_file_unsafe t kind =
+  let ext = Cm_kind.ext kind in
   let base = Obj_dir.cm_public_dir t.obj_dir kind in
   Path.relative base (t.obj_name ^ ext)
 
-let cm_public_file t ?ext (kind : Cm_kind.t) =
+let cm_public_file t (kind : Cm_kind.t) =
   match kind with
   | (Cmx | Cmo) when not (has_impl t) -> None
   |  Cmi when is_private t -> None
-  | _ -> Some (cm_public_file_unsafe t ?ext kind)
+  | _ -> Some (cm_public_file_unsafe t kind)
 
 let cmt_file t (kind : Ml_kind.t) =
   match kind with
diff --git a/src/module.mli b/src/module.mli
index 056ccd25f12..962d02c721e 100644
--- a/src/module.mli
+++ b/src/module.mli
@@ -113,8 +113,8 @@ val pp_flags : t -> (unit, string list) Build.t option
 
 val file            : t -> Ml_kind.t -> Path.t option
 val cm_source       : t -> Cm_kind.t -> Path.t option
-val cm_file         : t -> ?ext:string -> Cm_kind.t -> Path.t option
-val cm_public_file  : t -> ?ext:string -> Cm_kind.t -> Path.t option
+val cm_file         : t -> Cm_kind.t -> Path.t option
+val cm_public_file  : t -> Cm_kind.t -> Path.t option
 val cmt_file        : t -> Ml_kind.t -> Path.t option
 
 val obj_file : t -> kind:Cm_kind.t -> ext:string -> Path.t
@@ -123,10 +123,9 @@ val obj_name : t -> string
 
 (** Same as [cm_file] but doesn't raise if [cm_kind] is [Cmo] or [Cmx]
     and the module has no implementation.
-    If present [ext] replace the extension of the kind
  *)
-val cm_file_unsafe : t -> ?ext:string -> Cm_kind.t -> Path.t
-val cm_public_file_unsafe : t -> ?ext:string -> Cm_kind.t -> Path.t
+val cm_file_unsafe : t -> Cm_kind.t -> Path.t
+val cm_public_file_unsafe : t -> Cm_kind.t -> Path.t
 
 val odoc_file : t -> doc_dir:Path.Build.t -> Path.Build.t
 
diff --git a/src/virtual_rules.ml b/src/virtual_rules.ml
index d25bb77db35..f576f1f80f7 100644
--- a/src/virtual_rules.ml
+++ b/src/virtual_rules.ml
@@ -45,9 +45,9 @@ let setup_copy_rules_for_impl ~sctx ~dir vimpl =
   let modes =
     Dune_file.Mode_conf.Set.eval impl.modes
       ~has_native:(Option.is_some ctx.ocamlopt) in
-  let copy_obj_file ~src ~dst ?ext kind =
-    let src = Module.cm_file_unsafe src ?ext kind in
-    let dst = Module.cm_file_unsafe dst ?ext kind in
+  let copy_obj_file ~src ~dst kind =
+    let src = Module.cm_file_unsafe src kind in
+    let dst = Module.cm_file_unsafe dst kind in
     copy_to_obj_dir ~src ~dst:(Path.as_in_build_dir_exn dst) in
   let copy_objs src =
     let dst = Module.set_obj_dir ~obj_dir:(Obj_dir.of_local impl_obj_dir) src in
@@ -62,9 +62,13 @@ let setup_copy_rules_for_impl ~sctx ~dir vimpl =
     if Module.has_impl src then begin
       if modes.byte then
         copy_obj_file ~src ~dst Cmo;
-      if modes.native then
-        List.iter [Cm_kind.ext Cmx; ctx.ext_obj]
-          ~f:(fun ext -> copy_obj_file ~src ~dst ~ext Cmx)
+      if modes.native then begin
+        copy_obj_file ~src ~dst Cmx;
+        (let object_file = Module.obj_file ~kind:Cmx ~ext:ctx.ext_obj in
+         copy_to_obj_dir
+           ~src:(object_file src)
+           ~dst:(Path.as_in_build_dir_exn (object_file dst)))
+      end
     end
   in
   let copy_all_deps =