diff --git a/Straight_skeleton_2/include/CGAL/Straight_skeleton_2/Straight_skeleton_builder_traits_2_aux.h b/Straight_skeleton_2/include/CGAL/Straight_skeleton_2/Straight_skeleton_builder_traits_2_aux.h
index ecd71d6475f..7d9a0feb4e7 100644
--- a/Straight_skeleton_2/include/CGAL/Straight_skeleton_2/Straight_skeleton_builder_traits_2_aux.h
+++ b/Straight_skeleton_2/include/CGAL/Straight_skeleton_2/Straight_skeleton_builder_traits_2_aux.h
@@ -452,7 +452,7 @@ struct SS_converter : Converter
   
   Target_trisegment_2_ptr cvt_single_trisegment( Source_trisegment_2_ptr const& tri ) const
   {
-    CGAL_precondition( tri ) ;
+    CGAL_precondition( tri!= Source_trisegment_2_ptr() ) ;
     
     return Target_trisegment_2_ptr ( new Target_trisegment_2(cvt_s(tri->e0())
                                                             ,cvt_s(tri->e1())
diff --git a/Straight_skeleton_2/include/CGAL/Straight_skeleton_builder_2.h b/Straight_skeleton_2/include/CGAL/Straight_skeleton_builder_2.h
index 814c5b8ca20..12a6cfb28d5 100644
--- a/Straight_skeleton_2/include/CGAL/Straight_skeleton_builder_2.h
+++ b/Straight_skeleton_2/include/CGAL/Straight_skeleton_builder_2.h
@@ -355,7 +355,7 @@ private :
                                                                         
     CGAL_STSKEL_BUILDER_TRACE(5,"Trisegment for " << aTriedge << ":" << r ) ;
     
-    CGAL_postcondition_msg(r, "Unable to determine edges collinearity");
+    CGAL_postcondition_msg((r!= Trisegment_2_ptr()), "Unable to determine edges collinearity");
       
     return r ;  
   }