diff --git a/src/util/time-unit-test-clock.hpp b/src/util/time-unit-test-clock.hpp
index f099b84..7d67507 100644
--- a/src/util/time-unit-test-clock.hpp
+++ b/src/util/time-unit-test-clock.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).
  *
@@ -102,6 +102,9 @@
   nanoseconds m_currentTime;
 };
 
+extern template class UnitTestClock<system_clock>;
+extern template class UnitTestClock<steady_clock>;
+
 typedef UnitTestClock<system_clock> UnitTestSystemClock;
 typedef UnitTestClock<steady_clock> UnitTestSteadyClock;