diff --git a/lib/Slic3r/ExPolygon.pm b/lib/Slic3r/ExPolygon.pm
index 6dd78cf889..50c9debac0 100644
--- a/lib/Slic3r/ExPolygon.pm
+++ b/lib/Slic3r/ExPolygon.pm
@@ -201,6 +201,7 @@ sub medial_axis {
             $pointmap{$_->[A]} >= 2 && $pointmap{$_->[B]} >= 2
         } @skeleton_lines;
     }
+    return undef if !@skeleton_lines;
     
     # now build a single polyline
     my $polyline = [];