diff --git a/SceneGeom.cpp b/SceneGeom.cpp
index 829ecec..2869f39 100644
--- a/SceneGeom.cpp
+++ b/SceneGeom.cpp
@@ -271,7 +271,7 @@ Scene::DrawGeoObj( const IGdbIterator& iIter, int nPass, MdStMk siObj)
          Triangle3d Tria ;
          TriNormals3d TNrms ;
          int nId = pSTM->GetFirstTriangle( Tria) ;
-         while ( nId != -1) {
+         while ( nId != SVT_NULL) {
             pSTM->GetTriangleSmoothNormals( nId, TNrms) ;
             pGraphics->AddTriangle( Tria, TNrms) ;
             nId = pSTM->GetNextTriangle( nId, Tria) ;