diff --git a/src/amount.ml b/src/amount.ml
index 8692e015..79639201 100644
--- a/src/amount.ml
+++ b/src/amount.ml
@@ -1,6 +1,7 @@
 (* TODO
      have safe amount arithmetic
-     make type private *)
+     make type private
+     use config currency option *)
 (* Amounts of currency, serialized as `<Currency>:<IntegerPart>.<FractionalPart>`
      Fixed-precision numbers with 8 decimal places.
      - <Currency> must be at most 11 characters long
@@ -21,7 +22,7 @@ let ( let* ) o f = match o with Ok v -> f v | Error _ as e -> e
 
 let make ~sign ~currency ~value ~fraction =
   let value_upper_bound = Z.(pow (of_int 2) 52) |> Z.to_int64 in
-  let fraction_upper_bound = Int64.of_int 99_999_999 in
+  let fraction_upper_bound = Int64.of_int 100_000_000 in
   let* () = if value < Int64.zero then Error "value is negative" else Ok () in
   let* () =
     if fraction < Int64.zero then Error "fraction is negative" else Ok ()
@@ -31,7 +32,7 @@ let make ~sign ~currency ~value ~fraction =
     else Ok ()
   in
   let* () =
-    if fraction > fraction_upper_bound then
+    if fraction >= fraction_upper_bound then
       Error "fraction have more than 8 decimal digits"
     else Ok ()
   in
@@ -46,8 +47,6 @@ let to_string =
     in
     let pp_currency ppf = function `Eur -> string ppf "EUR" in
     fun ppf { sign; currency; value; fraction } ->
-      (* TODO amount
-         use config *)
       pf ppf "%a%a:%Ld.%02Ld" (Fmt.option pp_sign) sign pp_currency currency
         value fraction
   in