diff --git a/src/lib_shell/mempool_worker.ml b/src/lib_shell/mempool_worker.ml
index 48f756080..d59ee5cf5 100644
--- a/src/lib_shell/mempool_worker.ml
+++ b/src/lib_shell/mempool_worker.ml
@@ -149,7 +149,7 @@ module Make(Proto: Registered_protocol.T) : T with module Proto = Proto = struct
 
   module Request = struct
 
-    type 'a t = Validate : operation -> result t
+    type 'a t = Validate : operation -> result t [@@ocaml.unboxed]
 
     type view = View : _ t -> view
 
@@ -163,7 +163,7 @@ module Make(Proto: Registered_protocol.T) : T with module Proto = Proto = struct
         operation_encoding
 
     let pp ppf (View (Validate { hash })) =
-      Format.fprintf ppf "New parsed operation hash %a" Operation_hash.pp hash
+      Format.fprintf ppf "Validating new operation %a" Operation_hash.pp hash
   end
 
   module Event = struct