diff --git a/src/node/net/RPC.mli b/src/node/net/RPC.mli
index fa2ea2ddc..998936b2c 100644
--- a/src/node/net/RPC.mli
+++ b/src/node/net/RPC.mli
@@ -22,7 +22,7 @@ module Arg : sig
     name:string ->
     destruct:(string -> ('a, string) result) ->
     construct:('a -> string) ->
-    'a arg
+    unit -> 'a arg
 
   type descr = {
     name: string ;
diff --git a/src/node/shell/node_rpc_services.ml b/src/node/shell/node_rpc_services.ml
index dbed80a38..75d8bea6e 100644
--- a/src/node/shell/node_rpc_services.ml
+++ b/src/node/shell/node_rpc_services.ml
@@ -90,7 +90,7 @@ module Blocks = struct
       | `Test_prevalidation -> "test_prevalidation"
       | `Hash h -> Block_hash.to_b48check h in
     let destruct = parse_block in
-    RPC.Arg.make ~name ~descr ~construct ~destruct
+    RPC.Arg.make ~name ~descr ~construct ~destruct ()
 
   type preapply_param = {
     operations: Operation_hash.t list ;
@@ -341,7 +341,7 @@ module Operations = struct
     let destruct h =
       try Ok (Operation_hash.of_b48check h)
       with _ -> Error "Can't parse hash" in
-    RPC.Arg.make ~name ~descr ~construct ~destruct
+    RPC.Arg.make ~name ~descr ~construct ~destruct ()
 
   let bytes =
     RPC.service
diff --git a/src/proto/bootstrap/contract_repr.ml b/src/proto/bootstrap/contract_repr.ml
index d7e806669..3959aac3b 100644
--- a/src/proto/bootstrap/contract_repr.ml
+++ b/src/proto/bootstrap/contract_repr.ml
@@ -125,6 +125,7 @@ let arg =
     ~name: "contract_id"
     ~construct
     ~destruct
+    ()
 
 let compare l1 l2 =
   match l1, l2 with
diff --git a/src/proto/bootstrap/cycle_repr.ml b/src/proto/bootstrap/cycle_repr.ml
index e7f441277..3e2446574 100644
--- a/src/proto/bootstrap/cycle_repr.ml
+++ b/src/proto/bootstrap/cycle_repr.ml
@@ -22,6 +22,7 @@ let arg =
       ~name: "block_cycle"
       ~construct
       ~destruct
+      ()
 
 let pp ppf cycle = Format.fprintf ppf "%ld" cycle
 
diff --git a/src/proto/bootstrap/raw_level_repr.ml b/src/proto/bootstrap/raw_level_repr.ml
index 442f6be35..ab1d73b81 100644
--- a/src/proto/bootstrap/raw_level_repr.ml
+++ b/src/proto/bootstrap/raw_level_repr.ml
@@ -23,6 +23,7 @@ let arg =
     ~name: "block_level"
     ~construct
     ~destruct
+    ()
 
 let root = 0l
 let succ = Int32.succ
diff --git a/src/proto/bootstrap/services.ml b/src/proto/bootstrap/services.ml
index ee0f0c147..fc43a82f7 100644
--- a/src/proto/bootstrap/services.ml
+++ b/src/proto/bootstrap/services.ml
@@ -187,6 +187,7 @@ module Context = struct
         ~name: "public_key_hash"
         ~construct
         ~destruct
+        ()
 
     let pk_encoding =
       (obj2
diff --git a/src/proto/bootstrap/voting_period_repr.ml b/src/proto/bootstrap/voting_period_repr.ml
index 1db63ad18..39c9701a2 100644
--- a/src/proto/bootstrap/voting_period_repr.ml
+++ b/src/proto/bootstrap/voting_period_repr.ml
@@ -23,6 +23,7 @@ let arg =
     ~name: "voting_period"
     ~construct
     ~destruct
+    ()
 
 let root = 0l
 let succ = Int32.succ
diff --git a/src/proto/environment/RPC.mli b/src/proto/environment/RPC.mli
index e089d4295..7444f69e0 100644
--- a/src/proto/environment/RPC.mli
+++ b/src/proto/environment/RPC.mli
@@ -13,7 +13,7 @@ module Arg : sig
     name:string ->
     destruct:(string -> ('a, string) result) ->
     construct:('a -> string) ->
-    'a arg
+    unit -> 'a arg
 
   type descr = {
     name: string ;