diff --git a/src/aeso_ast_infer_types.erl b/src/aeso_ast_infer_types.erl
index d792ec9..3d9a760 100644
--- a/src/aeso_ast_infer_types.erl
+++ b/src/aeso_ast_infer_types.erl
@@ -1434,7 +1434,7 @@ create_bytes_constraints() ->
 get_bytes_constraints() ->
     ets_tab2list(bytes_constraints).
 
--spec add_bytes_constraint(byte_constraint()) -> ok.
+-spec add_bytes_constraint(byte_constraint()) -> true.
 add_bytes_constraint(Constraint) ->
     ets_insert(bytes_constraints, Constraint).
 
diff --git a/src/aeso_ast_to_fcode.erl b/src/aeso_ast_to_fcode.erl
index b648f65..3eedc24 100644
--- a/src/aeso_ast_to_fcode.erl
+++ b/src/aeso_ast_to_fcode.erl
@@ -1324,7 +1324,7 @@ pp_call(Fun, Args) ->
 pp_ftype(T) when is_atom(T) -> pp_text(T);
 pp_ftype(any) -> pp_text("_");
 pp_ftype({tvar, X}) -> pp_text(X);
-pp_ftype({bytes, N}) -> pp_text(["bytes(", integer_to_list(N), ")"]);
+pp_ftype({bytes, N}) -> pp_text("bytes(" ++ integer_to_list(N) ++ ")");
 pp_ftype({tuple, Ts}) ->
     pp_parens(pp_par(pp_punctuate(pp_text(","), [pp_ftype(T) || T <- Ts])));
 pp_ftype({list, T}) ->