diff --git a/ndn-cxx/transport/unix-transport.hpp b/ndn-cxx/transport/unix-transport.hpp
index c78b015..424ed53 100644
--- a/ndn-cxx/transport/unix-transport.hpp
+++ b/ndn-cxx/transport/unix-transport.hpp
@@ -29,9 +29,13 @@
 
 namespace ndn {
 
+namespace detail {
+
 template<typename BaseTransport, typename Protocol>
 class StreamTransportImpl;
 
+} // namespace detail
+
 /** \brief a transport using Unix stream socket
  */
 class UnixTransport : public Transport
@@ -74,7 +78,7 @@
 private:
   std::string m_unixSocket;
 
-  using Impl = StreamTransportImpl<UnixTransport, boost::asio::local::stream_protocol>;
+  using Impl = detail::StreamTransportImpl<UnixTransport, boost::asio::local::stream_protocol>;
   friend Impl;
   shared_ptr<Impl> m_impl;
 };