diff --git a/src/amount.ml b/src/amount.ml
index ab56d486..f5017805 100644
--- a/src/amount.ml
+++ b/src/amount.ml
@@ -2,7 +2,6 @@
    have a currency agnostic amount_lib.ml
    and specialize amount.ml to Config.currency??
 
-   no Zarith
    have safe amount arithmetic *)
 type sign =
   | Sign_plus
@@ -15,6 +14,7 @@ type t = {
   fraction: Int32.t;
 }
 
+(* rm Zarith *)
 let value_upper_bound = Z.(pow (of_int 2) 52) |> Z.to_int64
 
 (* TODO
@@ -24,14 +24,12 @@ let value_upper_bound = Z.(pow (of_int 2) 52) |> Z.to_int64
 let fraction_upper_bound = Int32.of_int 100_000_000
 
 let make ~sign ~currency ~value ~fraction =
-  let ( let* ) o f = match o with _e, false -> f () | e, true -> Error e in
-  let* () = ("value is negative", value < Int64.zero) in
-  let* () = ("fraction is negative", fraction < Int32.zero) in
-  let* () = ("value is greater than 2^52-1", value > value_upper_bound) in
-  let* () =
-    ("fraction has more than 8 decimal digits", fraction >= fraction_upper_bound)
-  in
-  Ok { sign; currency; value; fraction }
+  if value < Int64.zero then Error "value is negative"
+  else if fraction < Int32.zero then Error "fraction is negative"
+  else if value > value_upper_bound then Error "value is greater than 2^52-1"
+  else if fraction >= fraction_upper_bound then
+    Error "fraction has more than 8 decimal digits"
+  else Ok { sign; currency; value; fraction }
 
 module Parse = struct
   open Angstrom