diff --git a/VolZmapGraphics.cpp b/VolZmapGraphics.cpp
index 24c127f..09d998e 100644
--- a/VolZmapGraphics.cpp
+++ b/VolZmapGraphics.cpp
@@ -2294,8 +2294,8 @@ VolZmap::ExtMarchingCubes( int nBlock, VoxelContainer& vVox) const
          double dXMax = ( nDexMaxI + 0.5) * m_dStep ;
          double dYMax = ( nDexMaxJ + 0.5) * m_dStep ;            
         // Determino colore dei triangoli         
-         int nFaceInfGrade = m_Values[0][nDexMinJ * m_nNx[0] + nDexMinI][0].nToolMin ;
-         int nFaceSupGrade = m_Values[0][nDexMinJ * m_nNx[0] + nDexMinI][0].nToolMax ;     
+         int nFaceInfGrade = ( ! m_Values[0].empty() ? m_Values[0][nDexMinJ * m_nNx[0] + nDexMinI][0].nToolMin : 0) ;
+         int nFaceSupGrade = ( ! m_Values[0].empty() ? m_Values[0][nDexMinJ * m_nNx[0] + nDexMinI][0].nToolMax : 0) ;     
         // Piano di coordinata Z inferiore: versore normale rivolto come Z-  
          if ( nBlockIJK[2] == 0) {          
             Triangle3dEx trTria1, trTria2 ;