diff --git a/lib/Slic3r/TriangleMesh.pm b/lib/Slic3r/TriangleMesh.pm
index 216e28c1a..9e3f07223 100644
--- a/lib/Slic3r/TriangleMesh.pm
+++ b/lib/Slic3r/TriangleMesh.pm
@@ -23,8 +23,9 @@ use constant I_PREV_FACET_INDEX => 4;
 use constant I_NEXT_FACET_INDEX => 5;
 use constant I_FACET_EDGE       => 6;
 
-use constant FE_TOP             => 0;
-use constant FE_BOTTOM          => 1;
+use constant FE_NONE            => 0;
+use constant FE_TOP             => 1;
+use constant FE_BOTTOM          => 2;
 
 # always make sure BUILD is idempotent
 sub BUILD {
@@ -144,11 +145,11 @@ sub make_loops {
     # remove tangent edges
     {
         for (my $i = 0; $i <= $#lines; $i++) {
-            next unless defined $lines[$i] && defined $lines[$i][I_FACET_EDGE];
+            next unless defined $lines[$i] && $lines[$i][I_FACET_EDGE];
             # if the line is a facet edge, find another facet edge
             # having the same endpoints but in reverse order
             for (my $j = $i+1; $j <= $#lines; $j++) {
-                next unless defined $lines[$j] && defined $lines[$j][I_FACET_EDGE];
+                next unless defined $lines[$j] && $lines[$j][I_FACET_EDGE];
                 
                 # are these facets adjacent? (sharing a common edge on this layer)
                 if ($lines[$i][I_A_ID] == $lines[$j][I_B_ID] && $lines[$i][I_B_ID] == $lines[$j][I_A_ID]) {
@@ -156,7 +157,8 @@ sub make_loops {
                     # if they are both oriented upwards or downwards (like a 'V')
                     # then we can remove both edges from this layer since it won't 
                     # affect the sliced shape
-                    if ($lines[$j][I_FACET_EDGE] == $lines[$i][I_FACET_EDGE]) {
+                    if ($lines[$j][I_FACET_EDGE] &&
+                        $lines[$j][I_FACET_EDGE] == $lines[$i][I_FACET_EDGE]) {
                         $lines[$i] = undef;
                         $lines[$j] = undef;
                         last;
@@ -208,8 +210,8 @@ sub make_loops {
             if (0) {
                 require "Slic3r/SVG.pm";
                 Slic3r::SVG::output(undef, "same_point.svg",
-                    lines       => [ map $_->line, grep !defined $_->[I_FACET_EDGE], @lines ],
-                    red_lines   => [ map $_->line, grep defined $_->[I_FACET_EDGE], @lines ],
+                    lines       => [ map $_->line, grep !$_->[I_FACET_EDGE], @lines ],
+                    red_lines   => [ map $_->line, grep $_->[I_FACET_EDGE], @lines ],
                     points      => [ $self->vertices->[$point_id] ],
                     no_arrows => 0,
                 );
@@ -482,7 +484,7 @@ sub intersect_facet {
             $facet_id,                          # I_FACET_INDEX
             $prev_facet_index,                  # I_PREV_FACET_INDEX
             $next_facet_index,                  # I_NEXT_FACET_INDEX
-            undef,                              # I_FACET_EDGE
+            FE_NONE,                            # I_FACET_EDGE
             
             # Unused data:
             # a             => [$points[B][X], $points[B][Y]],