diff --git a/SceneGeom.cpp b/SceneGeom.cpp
index e382c18..1245e99 100644
--- a/SceneGeom.cpp
+++ b/SceneGeom.cpp
@@ -64,7 +64,7 @@ Scene::DrawGroup( int nId, int nPass, MdStMk cParent)
 {
   // creo un iteratore
    PtrOwner<IGdbIterator> pIter( CreateGdbIterator( m_pGeomDB)) ;
-   if ( ! ::IsValid( pIter))
+   if ( IsNull( pIter))
       return false ;
 
   // recupero il gruppo
@@ -93,7 +93,7 @@ Scene::DrawGroup( const IGdbIterator& iIter, int nPass, MdStMk cParent)
    }                                
   // creo un iteratore
    PtrOwner<IGdbIterator> pIter( CreateGdbIterator( m_pGeomDB)) ;
-   if ( ! ::IsValid( pIter))
+   if ( IsNull( pIter))
       return false ;
   // scandisco il gruppo
    for ( bool bNext = pIter->GoToFirstInGroup( iIter) ;
@@ -300,7 +300,7 @@ Scene::DeleteObjGraphicsGroup( int nId)
 {
   // creo un iteratore
    PtrOwner<IGdbIterator> pIter( CreateGdbIterator( m_pGeomDB)) ;
-   if ( ! ::IsValid( pIter))
+   if ( IsNull( pIter))
       return false ;
   // scandisco il gruppo
    bool bNext = pIter->GoToFirstInGroup( nId) ;