diff --git a/src/security/transform/base64-encode.hpp b/src/security/transform/base64-encode.hpp
index 7e9d99d..d62b5d8 100644
--- a/src/security/transform/base64-encode.hpp
+++ b/src/security/transform/base64-encode.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -78,7 +78,7 @@
 
 private:
   class Impl;
-  unique_ptr<Impl> m_impl;
+  const unique_ptr<Impl> m_impl;
 };
 
 unique_ptr<Transform>