diff --git a/lib/Slic3r/Geometry/Clipper.pm b/lib/Slic3r/Geometry/Clipper.pm
index d91b185ea..9dedc7a96 100644
--- a/lib/Slic3r/Geometry/Clipper.pm
+++ b/lib/Slic3r/Geometry/Clipper.pm
@@ -14,7 +14,7 @@ our $clipper = Math::Clipper->new;
 
 sub safety_offset {
     my ($polygons, $factor) = @_;
-    return Math::Clipper::offset($polygons, $factor || (scale 1e-05), 100000, JT_MITER, 2);
+    return Math::Clipper::offset($polygons, $factor // (scale 1e-05), 100000, JT_MITER, 2);
 }
 
 sub offset {
diff --git a/lib/Slic3r/Layer/Region.pm b/lib/Slic3r/Layer/Region.pm
index 3a1d2b0fa..2a2736719 100644
--- a/lib/Slic3r/Layer/Region.pm
+++ b/lib/Slic3r/Layer/Region.pm
@@ -118,11 +118,28 @@ sub make_surfaces {
 }
 
 sub _merge_loops {
-    my ($loops) = @_;
+    my ($loops, $safety_offset) = @_;
     
-    my $safety_offset = scale 0.1;
-    # merge everything
-    my $expolygons = [ map $_->offset_ex(-$safety_offset), @{union_ex(safety_offset($loops, $safety_offset))} ];
+    # Input loops are not suitable for evenodd nor nonzero fill types, as we might get
+    # two consecutive concentric loops having the same winding order - and we have to 
+    # respect such order. In that case, evenodd would create wrong inversions, and nonzero
+    # would ignore holes inside two concentric contours.
+    # So we're ordering loops and collapse consecutive concentric loops having the same 
+    # winding order.
+    # TODO: find a faster algorithm for this.
+    my @loops = sort { $a->encloses_point($b->[0]) ? 0 : 1 } @$loops;  # outer first
+    $safety_offset //= scale 0.1;
+    @loops = @{ safety_offset(\@loops, $safety_offset) };
+    my $expolygons = [];
+    while (my $loop = shift @loops) {
+        bless $loop, 'Slic3r::Polygon';
+        if ($loop->is_counter_clockwise) {
+            $expolygons = union_ex([ $loop, map @$_, @$expolygons ]);
+        } else {
+            $expolygons = diff_ex([ map @$_, @$expolygons ], [$loop]);
+        }
+    }
+    $expolygons = [ map $_->offset_ex(-$safety_offset), @$expolygons ];
     
     Slic3r::debugf "  %d surface(s) having %d holes detected from %d polylines\n",
         scalar(@$expolygons), scalar(map $_->holes, @$expolygons), scalar(@$loops);
diff --git a/t/loops.t b/t/loops.t
index c36505e4e..d0340b200 100644
--- a/t/loops.t
+++ b/t/loops.t
@@ -13,6 +13,12 @@ use Slic3r;
 use Slic3r::Test;
 
 {
+    # We only need to slice at one height, so we'll build a non-manifold mesh
+    # that still produces complete loops at that height. Triangular walls are 
+    # enough for this purpose.
+    # Basically we want to check what happens when three concentric loops happen
+    # to be at the same height, the two external ones being ccw and the other being
+    # a hole, thus cw.
     my (@vertices, @facets) = ();
     Slic3r::Test::add_facet($_, \@vertices, \@facets) for
         # external surface below the slicing Z
@@ -39,7 +45,7 @@ use Slic3r::Test;
     is scalar(@$loops), 3, 'correct number of loops detected';
     is scalar(grep $_->is_counter_clockwise, @$loops), 2, 'correct number of ccw loops detected';
     
-    my @surfaces = Slic3r::Layer::Region::_merge_loops($loops);
+    my @surfaces = Slic3r::Layer::Region::_merge_loops($loops, 0);
     is scalar(@surfaces), 1, 'one surface detected';
     is scalar(@{$surfaces[0]->expolygon})-1, 1, 'surface has one hole';
 }