diff --git a/lib/Slic3r/ExPolygon.pm b/lib/Slic3r/ExPolygon.pm
index 3e8d91713..b72b93542 100644
--- a/lib/Slic3r/ExPolygon.pm
+++ b/lib/Slic3r/ExPolygon.pm
@@ -170,9 +170,13 @@ sub _medial_axis_clip {
     return @result;
 }
 
+my $voronoi_lock :shared;
+
 sub _medial_axis_voronoi {
     my ($self, $width) = @_;
     
+    lock($voronoi_lock);
+    
     my $voronoi;
     {
         my @points = ();