diff --git a/src/api.ml b/src/api.ml
index dcbf036d..34acfc04 100644
--- a/src/api.ml
+++ b/src/api.ml
@@ -255,7 +255,7 @@ end
 
 let currency_specification =
   let open Config.Currency in
-  match Parse_config.Alt_unit_names.encode v.alt_unit_names with
+  match encode Alt_unit_names.jsont v.alt_unit_names with
   | Error e -> Fmt.failwith "json encoding error on alt_unit_names: %s." e
   | Ok alt_unit_names ->
       CurrencySpecification.
diff --git a/src/config.ml b/src/config.ml
index 6db64eed..a62866e9 100644
--- a/src/config.ml
+++ b/src/config.ml
@@ -87,6 +87,35 @@ module Exchangedb_postgres = struct
 end
 
 module Currency = struct
+  module Alt_unit_names = struct
+    open Syntax
+    module Int_map = Map.Make (Int)
+    module String_map = Map.Make (String)
+
+    type t = string Int_map.t
+
+    let jsont =
+      let of_string_map str_map =
+        str_map
+        |> String_map.to_list
+        |> list_map (fun (k, v) ->
+            match int_of_string_opt k with
+            | None -> Error "not an integer"
+            | Some k -> Ok (k, v))
+        |> function
+        | Error e -> Jsont.Error.msg Jsont.Meta.none e
+        | Ok l -> Int_map.of_list l
+      in
+      let to_string_map int_map =
+        int_map
+        |> Int_map.to_list
+        |> List.map (fun (k, v) -> (string_of_int k, v))
+        |> String_map.of_list
+      in
+      let string_map_jsont = Jsont.Object.as_string_map Jsont.string in
+      Jsont.map ~dec:of_string_map ~enc:to_string_map string_map_jsont
+  end
+
   type t = {
     enabled: [ `YES | `NO ];
     code: string;
@@ -94,7 +123,7 @@ module Currency = struct
     fractional_input_digits: int;
     fractional_normal_digits: int;
     fractional_trailing_zero_digits: int;
-    alt_unit_names: (int * string) list;
+    alt_unit_names: Alt_unit_names.t;
   }
 
   let currency_sections =
@@ -113,12 +142,13 @@ module Currency = struct
       fractional_trailing_zero_digits=
         get "fractional_trailing_zero_digits" |> int;
       alt_unit_names=
-        get "alt_unit_names" |> Alt_unit_names.decode |> Parse_config.unwrap;
+        get "alt_unit_names"
+        |> Jsont_bytesrw.decode_string Alt_unit_names.jsont
+        |> Parse_config.unwrap;
     }
 
   let all_currencies = List.map parse_currency currency_sections
 
-  (* I think the exchange only handle one currency *)
   let v =
     match
       List.find_opt (fun v -> v.code = Exchange.currency) all_currencies
diff --git a/src/parse_config.ml b/src/parse_config.ml
index 044873fb..83563448 100644
--- a/src/parse_config.ml
+++ b/src/parse_config.ml
@@ -212,34 +212,3 @@ let etag s =
       match Headers_lib.Etag.parse (Fmt.str "\"%s\"" s) with
       | Ok etag -> etag
       | Error _ -> fail "could not parse etag `%s`: %s" s e)
-
-(* TODO
-   move to another module
-   can we type the json as a Int_map directly? *)
-module Alt_unit_names = struct
-  open Syntax
-  module String_map = Map.Make (String)
-
-  let string_map_jsont = Jsont.Object.as_string_map Jsont.string
-
-  let decode s =
-    let* string_map = Jsont_bytesrw.decode_string string_map_jsont s in
-    let l = String_map.to_list string_map in
-    let* l =
-      list_map
-        (fun (k, v) ->
-          match int_of_string_opt k with
-          | None -> Error "alt_unit_names has a non-integer key"
-          | Some k -> Ok (k, v))
-        l
-    in
-    match List.find_opt (fun (i, _) -> i = 0) l with
-    | None -> Error "alt_unit_names with no entry for base value \"0\""
-    | Some _ -> Ok l
-
-  let encode l =
-    let l = List.map (fun (k, v) -> (string_of_int k, v)) l in
-    let string_map = String_map.of_list l in
-    let+ s = Jsont_bytesrw.encode_string string_map_jsont string_map in
-    s
-end
diff --git a/src/syntax.ml b/src/syntax.ml
index 9282b2dd..7db544ca 100644
--- a/src/syntax.ml
+++ b/src/syntax.ml
@@ -55,11 +55,3 @@ let list_fold_left f acc l =
       let* acc = acc in
       f acc v)
     (Ok acc) l
-
-let list_option l =
-  match (List.for_all Option.is_none l, List.for_all Option.is_some l) with
-  | _, true ->
-      let l = List.map Option.get l in
-      Ok (Some l)
-  | true, _ -> Ok None
-  | _, _ -> Error ()