diff --git a/src/binary_formats.ml b/src/binary_formats.ml
index fe66ef4e..6e1e8fc2 100644
--- a/src/binary_formats.ml
+++ b/src/binary_formats.ml
@@ -235,7 +235,7 @@ module Purpose = struct
     |+ field beint32 (fun t -> t.purpose)
     |> sealr
 
-  let make size purpose = { size= Int32.of_int size; purpose }
+  let make ~size purpose = { size= Int32.of_int size; purpose }
 end
 
 (* This is the running SHA512-hash over all
@@ -367,10 +367,8 @@ module Blinding_master_seed = MK_BASIC_32 ()
 (* Format used for to generate the signature on a request to withdraw
    coins from a reserve. *)
 module Withdraw_request_ps = struct
+  (* Purpose is #TALER_SIGNATURE_WALLET_RESERVE_WITHDRAW *)
   type t = {
-    (* TODO set it in encode/decode *)
-    (* Purpose is #TALER_SIGNATURE_WALLET_RESERVE_WITHDRAW *)
-    purpose: Purpose.t;
     (* Amount to withdraw, excluding fees, i.e.
        the total sum of the denominations of the coins.
        Note that the reserve must have a value of at least amount+fee. *)
@@ -419,20 +417,14 @@ module Withdraw_request_ps = struct
     + int32_size
     + Age_mask.size
 
+  let purpose = Purpose.make ~size Taler_signatures.wallet_reserve_withdraw
+
   let bin =
     let open Bin in
     record
-      (fun purpose amount fee h_planchets blinding_seed max_age_group mask ->
-        {
-          purpose;
-          amount;
-          fee;
-          h_planchets;
-          blinding_seed;
-          max_age_group;
-          mask;
-        })
-    |+ field Purpose.bin (fun t -> t.purpose)
+      (fun _purpose amount fee h_planchets blinding_seed max_age_group mask ->
+        { amount; fee; h_planchets; blinding_seed; max_age_group; mask })
+    |+ field Purpose.bin (Fun.const purpose)
     |+ field Amount.bin (fun t -> t.amount)
     |+ field Amount.bin (fun t -> t.fee)
     |+ field Hash_planchets_p.bin (fun t -> t.h_planchets)
@@ -462,7 +454,7 @@ module Withdraw_confirmation_ps = struct
   }
 
   let size = Purpose.size + Hash_planchets_p.size + int32_size
-  let purpose = Purpose.make size Taler_signatures.exchange_confirm_withdraw
+  let purpose = Purpose.make ~size Taler_signatures.exchange_confirm_withdraw
 
   let bin =
     let open Bin in