diff --git a/src/passes/4-typer/typer.ml b/src/passes/4-typer/typer.ml
index 8d6803534..7b3744d79 100644
--- a/src/passes/4-typer/typer.ml
+++ b/src/passes/4-typer/typer.ml
@@ -615,7 +615,7 @@ and type_expression : environment -> ?tv_opt:O.type_value -> I.expression -> O.a
       let output_type = body.type_annotation in
       return (E_lambda {binder = fst binder ; body}) (t_function input_type output_type ())
     )
-  | E_constant ( ("LIST_FOLD"|"MAP_FOLD"|"SET_FOLD") ,
+  | E_constant ( ("LIST_FOLD"|"MAP_FOLD"|"SET_FOLD") as opname ,
                   [ collect ; 
                     init_record ;
                     ( { expression = (I.E_lambda { binder = (name, None) ;
@@ -624,28 +624,28 @@ and type_expression : environment -> ?tv_opt:O.type_value -> I.expression -> O.a
                                                    result }) ;
                         location = _ }) as _lambda
                   ] ) ->
-      (* this special case is here force annotation of the lambda
+      (* this special case is here force annotation of the untyped lambda
          generated by pascaligo's for_collect loop *)
       let%bind lst' = bind_list @@ List.map (type_expression e) [collect ; init_record] in
       let tv_lst = List.map get_type_annotation lst' in
       let tv_col = List.nth tv_lst 0 in
       let tv_out = List.nth tv_lst 1 in
       let collect_inner_type = match tv_col.type_value' with
-        | O.T_constant ( ("list"|"set"|"map") , t) -> t
+        | O.T_constant ( ("list"|"set"|"map") , [t]) -> t
         | _ -> failwith "impossible" in
-      let input_type = t_tuple (tv_out::collect_inner_type) () in 
+      let input_type = t_tuple (tv_out::[collect_inner_type]) () in 
       let output_type = Some tv_out in
 
-      let e' = Environment.add_ez_binder name input_type e in
+      let e' = Environment.add_ez_binder "arguments" input_type e in
       let%bind body = type_expression ?tv_opt:output_type e' result in
       let output_type = body.type_annotation in
       let%bind lambda' = ok @@ make_a_e (E_lambda {binder = name ; body}) (t_function input_type output_type ()) e in
 
       let%bind lst' = ok @@ lst'@[lambda'] in
       let tv_lst = List.map get_type_annotation lst' in
-      let%bind (name', tv) =
-        type_constant name tv_lst tv_opt ae.location in
-      return (E_constant (name' , lst')) tv
+      let%bind (opname', tv) =
+        type_constant opname tv_lst tv_opt ae.location in
+      return (E_constant (opname' , lst')) tv
   | E_constant (name, lst) ->
       let%bind lst' = bind_list @@ List.map (type_expression e) lst in
       let tv_lst = List.map get_type_annotation lst' in