diff --git a/src/amount.ml b/src/amount.ml
index 9c10437c..c5e06082 100644
--- a/src/amount.ml
+++ b/src/amount.ml
@@ -84,7 +84,8 @@ let pp =
   in
   fun ppf { sign; currency; value; fraction } ->
     (* TODO
-       this depends on the currency's number of fraction digits *)
+       depends on the currency's number of fraction digits
+       assumes value and fraction are in bounds *)
     pf ppf "%a%s:%Ld.%02ld" (Fmt.option pp_sign) sign currency value fraction
 
 let to_string = Fmt.str "%a" pp