diff --git a/src/libslic3r/Fill/FillEnsuring.cpp b/src/libslic3r/Fill/FillEnsuring.cpp
index a64c0fd83..aa19bf09f 100644
--- a/src/libslic3r/Fill/FillEnsuring.cpp
+++ b/src/libslic3r/Fill/FillEnsuring.cpp
@@ -71,9 +71,11 @@ ThickPolylines make_fill_polylines(
         vertical_lines[i].a = Point{x, y_min};
         vertical_lines[i].b = Point{x, y_max};
     }
-    vertical_lines.push_back(vertical_lines.back());
-    vertical_lines.back().a = Point{coord_t(bb.min.x() + n_vlines * double(scaled_spacing) + scaled_spacing * 0.5), y_min};
-    vertical_lines.back().b = Point{vertical_lines.back().a.x(), y_max};
+    if (vertical_lines.size() > 0) {
+        vertical_lines.push_back(vertical_lines.back());
+        vertical_lines.back().a = Point{coord_t(bb.min.x() + n_vlines * double(scaled_spacing) + scaled_spacing * 0.5), y_min};
+        vertical_lines.back().b = Point{vertical_lines.back().a.x(), y_max};
+    }
 
     std::vector<std::vector<Line>> polygon_sections(n_vlines);