diff --git a/SimGear/simgear/scene/material/Effect.cxx b/SimGear/simgear/scene/material/Effect.cxx
index 2e10318..5c839d6 100644
--- a/SimGear/simgear/scene/material/Effect.cxx
+++ b/SimGear/simgear/scene/material/Effect.cxx
@@ -76,6 +76,14 @@
 #include <simgear/structure/SGExpression.hxx>
 #include <simgear/props/vectorPropTemplates.hxx>
 
+namespace osg
+{
+    std::size_t hash_value(osg::Shader::Type t)
+    {
+        return boost::hash_value(static_cast<int>(t));
+    }
+}
+
 
 namespace simgear
 {