diff --git a/src/binary_formats.ml b/src/binary_formats.ml
index f51dc198..a750e089 100644
--- a/src/binary_formats.ml
+++ b/src/binary_formats.ml
@@ -451,15 +451,23 @@ module WithdrawConfirmationPS = struct
     noreveal_index: int32;
   }
 
-  let size = Purpose.size + HashPlanchetsP.size + int32_size
-  let purpose = Purpose.make ~size Taler_signatures.exchange_confirm_withdraw
+  (*let size = Purpose.size + HashPlanchetsP.size + int32_size*)
+
+  open Bin
 
   let bin =
-    let open Bin in
-    record (fun _urpose h_planchets noreveal_index ->
-        { h_planchets; noreveal_index })
-    |+ field Purpose.bin (Fun.const purpose)
-    |+ field HashPlanchetsP.bin (fun t -> t.h_planchets)
-    |+ field beint32 (fun t -> t.noreveal_index)
+    record (fun purpose h_planchets noreveal_index ->
+        (purpose, { h_planchets; noreveal_index }))
+    |+ field Purpose.bin (fun (purpose, _t) -> purpose)
+    |+ field HashPlanchetsP.bin (fun (_p, t) -> t.h_planchets)
+    |+ field beint32 (fun (_p, t) -> t.noreveal_index)
     |> sealr
+
+  let size =
+    match Size.of_value (Size.size_of bin) with
+    | Dynamic _ | Unknown -> Fmt.failwith "size_of failure: size is not Static"
+    | Static n -> n
+
+  let purpose = Purpose.make ~size Taler_signatures.exchange_confirm_withdraw
+  let bin = map bin (fun (_p, t) -> t) (fun t -> (purpose, t))
 end
diff --git a/test/test.ml b/test/test.ml
index 5629d120..8f233513 100644
--- a/test/test.ml
+++ b/test/test.ml
@@ -80,8 +80,8 @@ let () =
   assert (size = size');
   assert (size = 76);
 
-  (* for cmp test with signatures.c output
+  (* for cmp test with signatures.c output *)
   let raw_str = Bin.to_string bin dummy_t in
   Printf.printf "%s" raw_str;
-   *)
+
   ()