diff --git a/websocketpp/common/thread.hpp b/websocketpp/common/thread.hpp
index b2ae135f68..e8abd035ae 100644
--- a/websocketpp/common/thread.hpp
+++ b/websocketpp/common/thread.hpp
@@ -37,6 +37,7 @@
 #ifdef _WEBSOCKETPP_CPP11_THREAD_
     #include <thread>
     #include <mutex>
+    #include <condition_variable>
 #else
     #include <boost/thread.hpp>
     #include <boost/thread/mutex.hpp>