diff --git a/Tunnel.h b/Tunnel.h
index 85590f58..f3c31461 100644
--- a/Tunnel.h
+++ b/Tunnel.h
@@ -91,7 +91,7 @@ namespace tunnel
 		Latency GetMeanLatency() const;
 
 		std::vector<Sample> m_samples;
-		std::mutex m_access;
+		mutable std::mutex m_access;
 	};
 	
 	class OutboundTunnel;