diff --git a/src/libslic3r/Geometry/VoronoiOffset.cpp b/src/libslic3r/Geometry/VoronoiOffset.cpp
index 46105220a4b..50be7587dca 100644
--- a/src/libslic3r/Geometry/VoronoiOffset.cpp
+++ b/src/libslic3r/Geometry/VoronoiOffset.cpp
@@ -796,7 +796,9 @@ void annotate_inside_outside(VD &vd, const Lines &lines)
             annotate_edge(&edge, EdgeCategory::PointsOutside);
             // Opposite edge of an infinite edge is certainly not active.
             annotate_edge(edge.twin(), edge.is_secondary() ? EdgeCategory::PointsToContour : EdgeCategory::PointsOutside);
-            annotate_vertex(edge.vertex0(), edge.is_secondary() ? VertexCategory::OnContour : VertexCategory::Outside);
+            if (edge.vertex0() != nullptr) {
+                annotate_vertex(edge.vertex0(), edge.is_secondary() ? VertexCategory::OnContour : VertexCategory::Outside);
+            } 
             // edge.vertex1() is null, it is implicitely outside.
             if (cell->contains_segment())
                 std::swap(cell, cell2);