diff --git a/lib/Slic3r.pm b/lib/Slic3r.pm
index ad6157ec11..30654bed79 100644
--- a/lib/Slic3r.pm
+++ b/lib/Slic3r.pm
@@ -79,6 +79,7 @@ use constant OVERLAP_FACTOR         => 1;
 use constant SMALL_PERIMETER_LENGTH => (6.5 / SCALING_FACTOR) * 2 * PI;
 use constant LOOP_CLIPPING_LENGTH_OVER_SPACING      => 0.15;
 use constant INFILL_OVERLAP_OVER_SPACING  => 0.45;
+use constant EXTERNAL_INFILL_MARGIN => 3;
 
 our $Config;
 
diff --git a/lib/Slic3r/Layer/Region.pm b/lib/Slic3r/Layer/Region.pm
index d773a9d036..aa758a8626 100644
--- a/lib/Slic3r/Layer/Region.pm
+++ b/lib/Slic3r/Layer/Region.pm
@@ -416,7 +416,7 @@ sub prepare_fill_surfaces {
 sub process_external_surfaces {
     my $self = shift;
     
-    my $margin = scale 3;  # TODO: ensure this is greater than the total thickness of the perimeters
+    my $margin = scale &Slic3r::EXTERNAL_INFILL_MARGIN;
     
     my @bottom = ();
     foreach my $surface (grep $_->surface_type == S_TYPE_BOTTOM, @{$self->fill_surfaces}) {
diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index 8431640428..99c73b4fff 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -572,6 +572,8 @@ sub discover_horizontal_shells {
     
     Slic3r::debugf "==> DISCOVERING HORIZONTAL SHELLS\n";
     
+    my $margin = scale &Slic3r::EXTERNAL_INFILL_MARGIN;
+    
     for my $region_id (0 .. ($self->print->regions_count-1)) {
         for (my $i = 0; $i < $self->layer_count; $i++) {
             my $layerm = $self->layers->[$i]->regions->[$region_id];
@@ -584,9 +586,13 @@ sub discover_horizontal_shells {
             
             EXTERNAL: foreach my $type (S_TYPE_TOP, S_TYPE_BOTTOM) {
                 # find slices of current type for current layer
-                # get both slices and fill_surfaces before the former contains the perimeters area
-                # and the latter contains the enlarged external surfaces
-                my $solid = [ map $_->expolygon, grep $_->surface_type == $type, @{$layerm->slices}, @{$layerm->fill_surfaces} ];
+                # use slices instead of fill_surfaces because they also include the perimeter area
+                # which needs to be propagated in shells; we need to grow slices like we did for
+                # fill_surfaces though.  Using both ungrown slices and grown fill_surfaces will
+                # not work in some situations, as there won't be any grown region in the perimeter 
+                # area (this was seen in a model where the top layer had one extra perimeter, thus
+                # its fill_surfaces was thinner than the lower layer's infill)
+                my $solid = [ map $_->expolygon->offset_ex($margin), grep $_->surface_type == $type, @{$layerm->slices} ];
                 next if !@$solid;
                 Slic3r::debugf "Layer %d has %s surfaces\n", $i, ($type == S_TYPE_TOP ? 'top' : 'bottom');