diff --git a/Straight_skeleton_2/include/CGAL/Straight_skeleton_2/Straight_skeleton_aux.h b/Straight_skeleton_2/include/CGAL/Straight_skeleton_2/Straight_skeleton_aux.h
index f7e26d01801..8e5bdfdd6d2 100644
--- a/Straight_skeleton_2/include/CGAL/Straight_skeleton_2/Straight_skeleton_aux.h
+++ b/Straight_skeleton_2/include/CGAL/Straight_skeleton_2/Straight_skeleton_aux.h
@@ -52,6 +52,8 @@ struct Segment_2_with_ID
   typedef typename K::Segment_2 Base;
   typedef typename K::Point_2 Point_2;
 
+  std::size_t id() const { return mID; }
+
 public:
   Segment_2_with_ID() : Base(), mID(-1) { }
   Segment_2_with_ID(Base const& aS) : Base(aS), mID(-1) { }