diff --git a/xs/src/libslic3r/Print.cpp b/xs/src/libslic3r/Print.cpp
index 1d186d3ab..b40430fec 100644
--- a/xs/src/libslic3r/Print.cpp
+++ b/xs/src/libslic3r/Print.cpp
@@ -463,7 +463,7 @@ Print::apply_config(DynamicPrintConfig config)
             
             std::vector<int> &region_volumes = object->region_volumes[region_id];
             for (std::vector<int>::const_iterator volume_id = region_volumes.begin(); volume_id != region_volumes.end(); ++volume_id) {
-                ModelVolume* volume = object->model_object()->volumes[*volume_id];
+                ModelVolume* volume = object->model_object()->volumes.at(*volume_id);
                 
                 PrintRegionConfig new_config = this->_region_config_from_model_volume(*volume);
                 
diff --git a/xs/src/libslic3r/Print.hpp b/xs/src/libslic3r/Print.hpp
index f0c1454fc..0e7334eaf 100644
--- a/xs/src/libslic3r/Print.hpp
+++ b/xs/src/libslic3r/Print.hpp
@@ -76,8 +76,10 @@ class PrintObject
     friend class Print;
 
     public:
-    // vector of (vectors of volume ids), indexed by region_id
-    std::vector<std::vector<int> > region_volumes;
+    // map of (vectors of volume ids), indexed by region_id
+    /* (we use map instead of vector so that we don't have to worry about
+       resizing it and the [] operator adds new items automagically) */
+    std::map< size_t,std::vector<int> > region_volumes;
     PrintObjectConfig config;
     t_layer_height_ranges layer_height_ranges;
     
diff --git a/xs/src/libslic3r/PrintObject.cpp b/xs/src/libslic3r/PrintObject.cpp
index 5fbca0c6d..1d33fbc00 100644
--- a/xs/src/libslic3r/PrintObject.cpp
+++ b/xs/src/libslic3r/PrintObject.cpp
@@ -10,8 +10,6 @@ PrintObject::PrintObject(Print* print, ModelObject* model_object, const Bounding
     _model_object(model_object),
     typed_slices(false)
 {
-    region_volumes.resize(this->_print->regions.size());
-
     // Compute the translation to be applied to our meshes so that we work with smaller coordinates
     {
         // Translate meshes so that our toolpath generation algorithms work with smaller
@@ -125,10 +123,6 @@ PrintObject::bounding_box(BoundingBox* bb) const
 void
 PrintObject::add_region_volume(int region_id, int volume_id)
 {
-    if (region_id >= region_volumes.size()) {
-        region_volumes.resize(region_id + 1);
-    }
-
     region_volumes[region_id].push_back(volume_id);
 }
 
@@ -369,7 +363,7 @@ PrintObject::bridge_over_infill()
             diff(internal_solid, to_bridge, &not_to_bridge, true);
             
             #ifdef SLIC3R_DEBUG
-            printf "Bridging %d internal areas at layer %d\n", scalar(@$to_bridge), $layer_id;
+            printf("Bridging %zu internal areas at layer %d\n", to_bridge.size(), layer->id());
             #endif
             
             // build the new collection of fill_surfaces