diff --git a/src/passes/8-typer-old/typer.ml b/src/passes/8-typer-old/typer.ml
index 8348653cf..b94a51475 100644
--- a/src/passes/8-typer-old/typer.ml
+++ b/src/passes/8-typer-old/typer.ml
@@ -88,6 +88,15 @@ module Errors = struct
       ("environment" , fun () -> Format.asprintf "%a" Environment.PP.full_environment e) ;
     ] in
     error ~data title message ()
+  
+  let michelson_or (c:I.constructor') loc () =
+    let title = (thunk "michelson_or types must be annotated") in
+    let message () = "" in
+    let data = [
+      ("constructor" , fun () -> Format.asprintf "%a" I.PP.constructor c);
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
 
   let wrong_arity (n:string) (expected:int) (actual:int) (loc : Location.t) () =
     let title () = "wrong arity" in
@@ -341,7 +350,10 @@ and evaluate_type (e:environment) (t:I.type_expression) : O.type_expression resu
         let%bind prev' = prev in
         let%bind v' = evaluate_type e v in
         let%bind () = match Environment.get_constructor k e with
-          | Some _ -> fail (redundant_constructor e k)
+          | Some _ ->
+            if I.CMap.mem (Constructor "M_left") m || I.CMap.mem (Constructor "M_right") m then
+              ok ()
+            else fail (redundant_constructor e k)
           | None -> ok () in
         ok @@ I.CMap.add k v' prev'
       in
@@ -477,6 +489,17 @@ and type_expression' : environment -> ?tv_opt:O.type_expression -> I.expression
         | None -> ok ()
         | Some tv' -> O.assert_type_expression_eq (tv' , ae.type_expression) in
       ok(ae)
+  | E_constructor {constructor = Constructor s ; element} when String.equal s "M_left" || String.equal s "M_right" -> (
+    let%bind t = trace_option (Errors.michelson_or (Constructor s) ae.location) @@ tv_opt in 
+    let%bind expr' = type_expression' e element in
+    ( match t.type_content with
+      | T_sum c ->
+        let ct = I.CMap.find (I.Constructor s) c in
+        let%bind _assert = O.assert_type_expression_eq (expr'.type_expression, ct) in
+        return (E_constructor {constructor = Constructor s; element=expr'}) t
+      | _ -> simple_fail "ll" 
+    )
+  )
   (* Sum *)
   | E_constructor {constructor; element} ->
       let%bind (c_tv, sum_tv) =