diff --git a/lib/Slic3r/Layer/Region.pm b/lib/Slic3r/Layer/Region.pm
index 75b62f486..3ce1603a1 100644
--- a/lib/Slic3r/Layer/Region.pm
+++ b/lib/Slic3r/Layer/Region.pm
@@ -150,7 +150,7 @@ sub _merge_loops {
         # of the loops, since the Orientation() function provided by Clipper
         # would do the same, thus repeating the calculation
         $slices = ($area[$i] >= 0)
-            ? union([ $loops->[$i], @$slices ])
+            ? [ $loops->[$i], @$slices ]
             : diff($slices, [$loops->[$i]]);
     }
     
diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index ca1560860..4493ee4a6 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -654,7 +654,7 @@ sub discover_horizontal_shells {
                                     offset($too_narrow, +$margin),
                                     [ map $_->p, @neighbor_fill_surfaces ],
                                 )};
-                                $new_internal_solid = $solid = union([ @grown, @$new_internal_solid ]);
+                                $new_internal_solid = $solid = [ @grown, @$new_internal_solid ];
                             } else {
                                 # if we're printing a hollow object, we discard such small parts
                                 $new_internal_solid = $solid = diff(