diff --git a/dune-project b/dune-project
index 5bd4e1f1..1aadd32c 100644
--- a/dune-project
+++ b/dune-project
@@ -34,7 +34,6 @@
    fmt
    bin
    angstrom
-   zarith
    mirage-crypto
    digestif
    duration
diff --git a/mte.opam b/mte.opam
index 8bdbc121..53820528 100644
--- a/mte.opam
+++ b/mte.opam
@@ -21,7 +21,6 @@ depends: [
   "fmt"
   "bin"
   "angstrom"
-  "zarith"
   "mirage-crypto"
   "digestif"
   "duration"
diff --git a/src/amount.ml b/src/amount.ml
index f5017805..9c10437c 100644
--- a/src/amount.ml
+++ b/src/amount.ml
@@ -14,8 +14,7 @@ type t = {
   fraction: Int32.t;
 }
 
-(* rm Zarith *)
-let value_upper_bound = Z.(pow (of_int 2) 52) |> Z.to_int64
+let value_upper_bound = Int64.of_float @@ Float.pow 2. 52.
 
 (* TODO
    the constraint is on the number of digits,
@@ -97,28 +96,31 @@ let jsont = Jsont.of_of_string ~kind:"Amount" of_string ~enc:to_string
 (* byte length of currency string *)
 let currency_len = 12
 
-(* TODO error *)
-let pad_currency s =
+let pad_currency_string s =
   let len = String.length s in
-  assert (len <= 11);
+  assert (len < currency_len);
   let b = Bytes.make 12 '\x00' in
   Bytes.blit_string s 0 b 0 len;
   Bytes.to_string b
 
+(* binary decoding unused? *)
+let make_exn value fraction currency =
+  match make ~sign:None ~currency ~value ~fraction with
+  | Error _ -> Fmt.failwith "Amount of binary data failure"
+  | Ok v -> v
+
 let bin =
   let open Bin in
-  record (fun _value _fraction _currency ->
-      (* no need to decode amount? *)
-      assert false)
+  record make_exn
   |+ field neint64 (fun t -> t.value)
   |+ field neint32 (fun t -> t.fraction)
-  |+ field (bytes currency_len) (fun t -> pad_currency t.currency)
+  |+ field (bytes currency_len) (fun t -> pad_currency_string t.currency)
   |> sealr
 
 let bin_nbo =
   let open Bin in
-  record (fun _value _fraction _currency -> assert false)
+  record make_exn
   |+ field beint64 (fun t -> t.value)
   |+ field beint32 (fun t -> t.fraction)
-  |+ field (bytes currency_len) (fun t -> pad_currency t.currency)
+  |+ field (bytes currency_len) (fun t -> pad_currency_string t.currency)
   |> sealr