diff --git a/include/osg/Drawable b/include/osg/Drawable
index adec13f33..e17ecd98f 100644
--- a/include/osg/Drawable
+++ b/include/osg/Drawable
@@ -172,7 +172,7 @@ class SG_EXPORT Drawable : public Object
         void compile(State& state);
 
 
-        struct AppCallback : public osg::Referenced
+        struct AppCallback : public virtual osg::Referenced
         {
                 /** do customized app code.*/
                 virtual void app(osg::NodeVisitor *visitor, osg::Drawable* drawable) = 0;
@@ -188,7 +188,7 @@ class SG_EXPORT Drawable : public Object
         const AppCallback* getAppCallback() const { return _appCallback.get(); }
 
 
-        struct CullCallback : public osg::Referenced
+        struct CullCallback : public virtual osg::Referenced
         {
                 /** do customized cull code.*/
                 virtual bool cull(osg::NodeVisitor *visitor, osg::Drawable* drawable, osg::State *state=NULL) const = 0;
@@ -209,7 +209,7 @@ class SG_EXPORT Drawable : public Object
           * the user intends to decorate the exist draw code then simple call the drawable->drawImplementation() from
           * with the callbacks drawImplementation() method.  This allows the users to do both pre and post callbacks
           * without fuss and can even diable the inner draw in required.*/
-        struct DrawCallback : public osg::Referenced
+        struct DrawCallback : public virtual osg::Referenced
         {
             /** do customized draw code.*/
             virtual void drawImplementation(State& state,const osg::Drawable* drawable) const = 0;
diff --git a/include/osg/NodeCallback b/include/osg/NodeCallback
index 1821eaca4..e278ec507 100644
--- a/include/osg/NodeCallback
+++ b/include/osg/NodeCallback
@@ -13,7 +13,7 @@ namespace osg {
 class Node;
 class NodeVisitor;
 
-class SG_EXPORT NodeCallback : public Referenced {
+class SG_EXPORT NodeCallback : public virtual Referenced {
 
     public :
     
diff --git a/include/osg/PositionAttitudeTransform b/include/osg/PositionAttitudeTransform
index c37e0f479..59a6c7572 100644
--- a/include/osg/PositionAttitudeTransform
+++ b/include/osg/PositionAttitudeTransform
@@ -53,7 +53,7 @@ class SG_EXPORT PositionAttitudeTransform : public Transform
         /** Callback which can be attached to a PositionAttitudeTransform
           * as an app callback to allow it to follow the path defined by a
           * AnimationPath.*/
-        class SG_EXPORT AnimationPathCallback : public NodeCallback
+        class SG_EXPORT AnimationPathCallback : public virtual NodeCallback
         {
             public:
             
diff --git a/include/osg/Transform b/include/osg/Transform
index 6841535d5..b37020fed 100644
--- a/include/osg/Transform
+++ b/include/osg/Transform
@@ -73,7 +73,7 @@ class SG_EXPORT Transform : public Group
         /** Callback attached to an Transform to specify how to compute the
           * modelview transformation for the transform below the Transform
           * node. */
-        struct ComputeTransformCallback : public osg::Referenced
+        struct ComputeTransformCallback : public virtual osg::Referenced
         {
             /** Get the transformation matrix which moves from local coords
               * to world coords.*/