diff --git a/include/osgTerrain/ValidDataOperator b/include/osgTerrain/ValidDataOperator
index 799a9a593..8f7410492 100644
--- a/include/osgTerrain/ValidDataOperator
+++ b/include/osgTerrain/ValidDataOperator
@@ -14,6 +14,7 @@
 #ifndef OSGTERRAIN_VALIDDATAOPERATOR
 #define OSGTERRAIN_VALIDDATAOPERATOR 1
 
+#include <osg/Referenced>
 #include <osgTerrain/Export>
 
 namespace osgTerrain {