diff --git a/xs/src/libslic3r/LayerRegion.cpp b/xs/src/libslic3r/LayerRegion.cpp
index 773801c98..3f36d3f68 100644
--- a/xs/src/libslic3r/LayerRegion.cpp
+++ b/xs/src/libslic3r/LayerRegion.cpp
@@ -169,7 +169,7 @@ LayerRegion::process_external_surfaces()
         fill_boundaries = SurfaceCollection(surfaces);
     } else {
         for (const Surface &s : surfaces)
-            if (s.is_external())
+            if (s.surface_type != stInternal)
                 fill_boundaries.surfaces.push_back(s);
     }
     
@@ -238,7 +238,7 @@ LayerRegion::prepare_fill_surfaces()
     
     // if no solid layers are requested, turn top/bottom surfaces to internal
     if (this->region()->config.top_solid_layers == 0 && this->region()->config.min_top_bottom_shell_thickness <= 0) {
-        for (Surfaces::iterator surface = this->fill_surfaces.surfaces.begin(); surface != this->fill_surfaces.surfaces.end(); ++surface) {
+        for (Surface &surface : this->fill_surfaces.surfaces) {
             if (surface->surface_type == stTop) {
                 if (this->layer()->object()->config.infill_only_where_needed) {
                     surface->surface_type = (stInternal | stVoid);
@@ -250,7 +250,7 @@ LayerRegion::prepare_fill_surfaces()
     }
     
     if (this->region()->config.bottom_solid_layers == 0 && this->region()->config.min_top_bottom_shell_thickness <= 0) {
-        for (Surfaces::iterator surface = this->fill_surfaces.surfaces.begin(); surface != this->fill_surfaces.surfaces.end(); ++surface) {
+        for (Surface &surface : this->fill_surfaces.surfaces) {
             if (surface->is_bottom())
                 surface->surface_type = stInternal;
         }