diff --git a/src/encoding/tlv.cpp b/src/encoding/tlv.cpp
index 4de73a1..712291c 100644
--- a/src/encoding/tlv.cpp
+++ b/src/encoding/tlv.cpp
@@ -1,5 +1,5 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
+/*
  * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
@@ -25,7 +25,7 @@
 namespace tlv {
 
 std::ostream&
-operator<<(std::ostream& os, const SignatureTypeValue& signatureType)
+operator<<(std::ostream& os, SignatureTypeValue signatureType)
 {
   switch (signatureType) {
     case SignatureTypeValue::DigestSha256:
diff --git a/src/encoding/tlv.hpp b/src/encoding/tlv.hpp
index 49d61e5..52115f9 100644
--- a/src/encoding/tlv.hpp
+++ b/src/encoding/tlv.hpp
@@ -96,7 +96,7 @@
   AppPrivateBlock2 = 32767
 };
 
-enum SignatureTypeValue {
+enum SignatureTypeValue : uint16_t {
   DigestSha256 = 0,
   SignatureSha256WithRsa = 1,
   // <Unassigned> = 2,
@@ -104,7 +104,7 @@
 };
 
 std::ostream&
-operator<<(std::ostream& os, const SignatureTypeValue& signatureType);
+operator<<(std::ostream& os, SignatureTypeValue signatureType);
 
 /** @brief TLV-TYPE numbers for SignatureInfo features
  *  @sa docs/tutorials/certificate-format.rst