diff --git a/lib/bap_sema/bap_sema_flatten.ml b/lib/bap_sema/bap_sema_flatten.ml
index 17a5e0404..677d08cc9 100644
--- a/lib/bap_sema/bap_sema_flatten.ml
+++ b/lib/bap_sema/bap_sema_flatten.ml
@@ -94,14 +94,28 @@ let flatten_exp
       Bil.Var var, insert blk def
     | Bil.Extract (n, p, x) ->
       aux x blk >>= fun (x, blk) ->
-      new_var (Var.sort_of_typ (get_direct_typ x)) >>= fun var ->
+      let res_typ = Type.Imm (n - p + 1) in
+      new_var (Var.sort_of_typ (res_typ)) >>= fun var ->
       let e = Bil.Extract (n, p, x) in
       new_def var e >>| fun def ->
       Bil.Var var, insert blk def
     | Bil.Concat (x, y) ->
       aux x blk >>= fun (x, blk) ->
       aux y blk >>= fun (y, blk) ->
-      new_var (Var.sort_of_typ (get_direct_typ y)) >>= fun var ->
+      let exp_size (e : exp) : int option = 
+        let typ_size (typ : Type.t) : (int option) = match typ with
+          | Type.Imm i -> Some i
+          | Type.Unk -> None 
+          | _ -> failwith "concating non-immediates"
+        in
+        match e with
+        | Bil.Var v -> typ_size (Var.typ v)
+        | Bil.Unknown (_, t) ->  typ_size t
+        | Bil.Int w -> Some (Word.bitwidth w)
+        | _ -> failwith "the expression is not flattened"
+      in
+      let res_typ =  Option.both (exp_size x) (exp_size y) |> Option.value_map ~default:Type.Unk ~f:(fun (x_size, y_size) -> Type.Imm (x_size + y_size)) in
+      new_var (Var.sort_of_typ (res_typ)) >>= fun var ->
       let e = Bil.Concat (x, y) in
       new_def var e >>| fun def ->
       Bil.Var var, insert blk def in