diff --git a/proto.go b/proto.go
index 9b99401..6c2ae1c 100644
--- a/proto.go
+++ b/proto.go
@@ -138,7 +138,7 @@ func (c *protoCodecV2) Marshal(d []byte, v interface{}, opts ...codec.Option) ([
 				marshalOptions = f
 			}
 		}
-		return marshalOptions.MarshalAppend(d, m)
+		return marshalOptions.MarshalAppend(d[:0], m)
 	default:
 		return nil, codec.ErrInvalidMessage
 	}