diff --git a/Nef_2/include/CGAL/Nef_2/PM_const_decorator.h b/Nef_2/include/CGAL/Nef_2/PM_const_decorator.h
index 7748f4b1f61..de2eb257997 100644
--- a/Nef_2/include/CGAL/Nef_2/PM_const_decorator.h
+++ b/Nef_2/include/CGAL/Nef_2/PM_const_decorator.h
@@ -481,6 +481,8 @@ check_integrity_and_topological_planarity(bool faces) const
     first=false;
   }
 
+  CGAL_assertion(iso_vert_num == iv_num);
+
   std::size_t v_num = number_of_vertices() - iso_vert_num;
   std::size_t e_num = number_of_edges();
   std::size_t c_num = number_of_connected_components() - iso_vert_num;