diff --git a/src/libraries/datatype/unmarshal.ml b/src/libraries/datatype/unmarshal.ml
index df6726ec62cf6d0fe9da6e13ae9dfa59fe2829c0..314dadf1be0bc74b5a431dddd1ef5192f0376eb6 100644
--- a/src/libraries/datatype/unmarshal.ml
+++ b/src/libraries/datatype/unmarshal.ml
@@ -503,7 +503,7 @@ let input_val ch t =
           read_string stk t len
 
       | _ ->
-	  ill_formed (Printf.sprintf "code %x" code)
+	  ill_formed (Printf.sprintf "code 0x%x" code)
     in
     match t with
     | Dynamic f ->