diff --git a/RotationMinimizingFrame.cpp b/RotationMinimizingFrame.cpp
index cbb878c..c81d7dc 100644
--- a/RotationMinimizingFrame.cpp
+++ b/RotationMinimizingFrame.cpp
@@ -92,11 +92,18 @@ RotationMinimizingFrame::GetFrameAtParam( const Frame3d& frAct, const double dPa
    Vector3d vtCurrR = frAct.VersX() ;
    Vector3d vtCurrT = frAct.VersZ() ;
 
-  // punto i-esimo sulla curva e suo vettore tangente
+  // punto i-esimo sulla curva e suo vettore tangente medio
+   Point3d ptNextM, ptNextP ;
+   Vector3d vtNextM, vtNextP ;
+   if ( ! m_pCrv->GetPointD1D2( dParNext, ICurve::FROM_MINUS, ptNextM, &vtNextM)  ||
+        ! m_pCrv->GetPointD1D2( dParNext, ICurve::FROM_PLUS, ptNextP, &vtNextP)  ||
+        ! vtNextM.Normalize()  ||  ! vtNextP.Normalize())
+      return false ;
    Point3d ptNext ;
    Vector3d vtNextT ;
-   if ( ! m_pCrv->GetPointD1D2( dParNext, ICurve::FROM_MINUS, ptNext, &vtNextT)  ||
-        ! vtNextT.Normalize())
+   ptNext = Media( ptNextM, ptNextP) ;
+   vtNextT = Media( vtNextM, vtNextP) ;
+   if ( ! vtNextT.Normalize())
       return false ;
 
   // controllo per casi degeneri
diff --git a/StmFromCurves.cpp b/StmFromCurves.cpp
index 8ccb23d..3ffa6ee 100644
--- a/StmFromCurves.cpp
+++ b/StmFromCurves.cpp
@@ -1095,7 +1095,7 @@ GetSurfTriMeshSwept3d( const ICurve* pSect, const ICurve* pGuide, const Vector3d
   // determino se la guida è chiusa
    bool bGuideClosed = pGuide->IsClosed() ;
   // determino algoritmo da usare per calcolare i riferimenti lungo la curva
-   bool bRMF = vtAx.IsSmall() ;
+   bool bRMF = ( ! vtAx.IsValid()  ||  vtAx.IsSmall()) ;
 
   // riferimento all'inizio della linea guida
    Point3d ptStart ;
@@ -1242,9 +1242,9 @@ GetSurfTriMeshSwept( const ICurve* pSect, const ICurve* pGuide, const Vector3d&
       if ( pCompo != nullptr  &&  pCompo->IsALine( 10 * EPS_SMALL, ptStart, ptEnd))
          bIsLine = true ;
    }
-  // se la guida è piana
+  // se la guida è piana e il vettore di riferimento è non definito oppure non nullo
    Plane3d plGuide ;
-   if ( pGuide->IsFlat( plGuide, bIsLine, 10 * EPS_SMALL))
+   if ( pGuide->IsFlat( plGuide, bIsLine, 10 * EPS_SMALL)  &&  ( ! vtAx.IsValid()  ||  ! vtAx.IsSmall()))
       return GetSurfTriMeshSweptInPlane( pSect, pGuide, plGuide.GetVersN(), bCapEnds, dLinTol) ;
 
   // altrimenti swept 3d