diff --git a/include/osgUtil/RenderBin b/include/osgUtil/RenderBin
index c0f3fd85e..de70983a5 100644
--- a/include/osgUtil/RenderBin
+++ b/include/osgUtil/RenderBin
@@ -72,7 +72,7 @@ class OSGUTIL_EXPORT RenderBin : public osg::Object
         
         struct SortCallback : public osg::Referenced    
         {
-            virtual void sort(RenderBin*) {};
+            virtual void sort(RenderBin*) = 0;
         };
 
         void setSortLocalCallback(SortCallback* sortCallback) { _sortLocalCallback = sortCallback; }