diff --git a/src/amount.ml b/src/amount.ml
index 79639201..1fb66513 100644
--- a/src/amount.ml
+++ b/src/amount.ml
@@ -19,22 +19,19 @@ type t = {
 }
 
 let ( let* ) o f = match o with Ok v -> f v | Error _ as e -> e
+let check_not msg = function false -> Ok () | true -> Error msg
+let value_upper_bound = Z.(pow (of_int 2) 52) |> Z.to_int64
+let fraction_upper_bound = Int64.of_int 100_000_000
 
 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 100_000_000 in
-  let* () = if value < Int64.zero then Error "value is negative" else Ok () in
+  let* () = check_not "value is negative" (value < Int64.zero) in
+  let* () = check_not "fraction is negative" (fraction < Int64.zero) in
   let* () =
-    if fraction < Int64.zero then Error "fraction is negative" else Ok ()
+    check_not "value is greater than 2^52" (value > value_upper_bound)
   in
   let* () =
-    if value > value_upper_bound then Error "value is greater than 2^52"
-    else Ok ()
-  in
-  let* () =
-    if fraction >= fraction_upper_bound then
-      Error "fraction have more than 8 decimal digits"
-    else Ok ()
+    check_not "fraction has more than 8 decimal digits"
+      (fraction >= fraction_upper_bound)
   in
   Ok { sign; currency; value; fraction }