diff --git a/src/proto_alpha/lib_protocol/src/operation_repr.ml b/src/proto_alpha/lib_protocol/src/operation_repr.ml
index c0755c6fb..478bbde8e 100644
--- a/src/proto_alpha/lib_protocol/src/operation_repr.ml
+++ b/src/proto_alpha/lib_protocol/src/operation_repr.ml
@@ -642,11 +642,11 @@ let () =
     (fun () -> Missing_signature)
 
 let check_signature (type kind) key ({ shell ; protocol_data } : kind operation) =
-  let check ?watermark contents signature =
+  let check ~watermark contents signature =
     let unsigned_operation =
       Data_encoding.Binary.to_bytes_exn
         unsigned_operation_encoding (shell, contents) in
-    if Signature.check ?watermark key signature unsigned_operation then
+    if Signature.check ~watermark key signature unsigned_operation then
       return ()
     else
       fail Invalid_signature in