diff --git a/src/osgTerrain/DataSet.cpp b/src/osgTerrain/DataSet.cpp
index 92375e6b8..0b7733f6d 100644
--- a/src/osgTerrain/DataSet.cpp
+++ b/src/osgTerrain/DataSet.cpp
@@ -3842,7 +3842,10 @@ struct DerefLessFunctor
 {
     bool operator () (const T& lhs, const T& rhs)
     {
-        return (lhs.valid() && rhs.valid()) && (lhs->getSortValue() > rhs->getSortValue());
+        if (!lhs || !rhs) return lhs<rhs;
+        if (lhs->getLayer() < rhs->getLayer()) return true;
+        if (rhs->getLayer() < lhs->getLayer()) return false;
+        return (lhs->getSortValue() > rhs->getSortValue());
     }
 };