diff --git a/TransitTunnel.cpp b/TransitTunnel.cpp
index 31cbb4f8..770ab246 100644
--- a/TransitTunnel.cpp
+++ b/TransitTunnel.cpp
@@ -74,6 +74,7 @@ namespace tunnel
 
 	void TransitTunnelGateway::FlushTunnelDataMsgs ()
 	{
+		std::unique_lock<std::mutex> l(m_SendMutex);
 		m_Gateway.SendBuffer ();
 	}