diff --git a/simgear/scene/material/matmodel.cxx b/simgear/scene/material/matmodel.cxx
index c45446e0..2767322a 100644
--- a/simgear/scene/material/matmodel.cxx
+++ b/simgear/scene/material/matmodel.cxx
@@ -28,7 +28,7 @@
 #include <simgear/compiler.h>
 
 #include <map>
-
+#include <mutex>
 
 #include <osg/AlphaFunc>
 #include <osg/Group>