diff --git a/lib/Slic3r/GUI/Plater.pm b/lib/Slic3r/GUI/Plater.pm
index aa6a521e..6b818ed6 100644
--- a/lib/Slic3r/GUI/Plater.pm
+++ b/lib/Slic3r/GUI/Plater.pm
@@ -403,7 +403,7 @@ sub load_model_objects {
         );
         push @obj_idx, $#{ $self->{objects} };
     
-        if (!defined $model_object->instances) {
+        if ($model_object->instances_count == 0) {
             # if object has no defined position(s) we need to rearrange everything after loading
             $need_arrange = 1;
         
diff --git a/lib/Slic3r/Print.pm b/lib/Slic3r/Print.pm
index 050e4c39..cdf26c9e 100644
--- a/lib/Slic3r/Print.pm
+++ b/lib/Slic3r/Print.pm
@@ -104,8 +104,9 @@ sub apply_config {
     if ($rearrange_regions) {
         # the current subdivision of regions does not make sense anymore.
         # we need to remove all objects and re-add them
+        my @model_objects = map $_->model_object, @{$self->objects};
         $self->clear_objects;
-        $self->add_model_object($_->model_object) for @{$self->objects};
+        $self->add_model_object($_) for @model_objects;
     }
 }
 
@@ -137,7 +138,7 @@ sub add_model_object {
         $config->apply_dynamic($object_config);
         
         if (defined $volume->material_id) {
-            my $material_config = $object->model->get_material($volume->material_id)->config->clone;
+            my $material_config = $volume->material->config->clone;
             $material_config->normalize;
             $config->apply_dynamic($material_config);
         }
diff --git a/xs/src/Model.cpp b/xs/src/Model.cpp
index 35cf3379..5d24b39b 100644
--- a/xs/src/Model.cpp
+++ b/xs/src/Model.cpp
@@ -321,8 +321,31 @@ ModelVolume::ModelVolume(ModelObject* object, const TriangleMesh &mesh)
 {}
 
 ModelVolume::ModelVolume(ModelObject* object, const ModelVolume &other)
-:   object(object), material_id(other.material_id), mesh(other.mesh), modifier(other.modifier)
-{}
+:   object(object), mesh(other.mesh), modifier(other.modifier)
+{
+    this->material_id(other.material_id());
+}
+
+t_model_material_id
+ModelVolume::material_id() const
+{
+    return this->_material_id;
+}
+
+void
+ModelVolume::material_id(t_model_material_id material_id)
+{
+    this->_material_id = material_id;
+    
+    // ensure this->_material_id references an existing material
+    (void)this->object->get_model()->add_material(material_id);
+}
+
+ModelMaterial*
+ModelVolume::material() const
+{
+    return this->object->get_model()->get_material(this->_material_id);
+}
 
 #ifdef SLIC3RXS
 REGISTER_CLASS(ModelVolume, "Model::Volume");
diff --git a/xs/src/Model.hpp b/xs/src/Model.hpp
index 84c68ee6..9e2d1234 100644
--- a/xs/src/Model.hpp
+++ b/xs/src/Model.hpp
@@ -138,14 +138,17 @@ class ModelVolume
 {
     friend class ModelObject;
     public:
-    t_model_material_id material_id;
     TriangleMesh mesh;
     bool modifier;
     
     ModelObject* get_object() const { return this->object; };
+    t_model_material_id material_id() const;
+    void material_id(t_model_material_id material_id);
+    ModelMaterial* material() const;
     
     private:
     ModelObject* object;
+    t_model_material_id _material_id;
     
     ModelVolume(ModelObject *object, const TriangleMesh &mesh);
     ModelVolume(ModelObject *object, const ModelVolume &other);
diff --git a/xs/xsp/Model.xsp b/xs/xsp/Model.xsp
index 1dc8d010..5903994c 100644
--- a/xs/xsp/Model.xsp
+++ b/xs/xsp/Model.xsp
@@ -162,10 +162,10 @@ ModelMaterial::attributes()
     Ref<ModelObject> object()
         %code%{ RETVAL = THIS->get_object(); %};
     
-    t_model_material_id material_id()
-        %code%{ RETVAL = THIS->material_id; %};
+    t_model_material_id material_id();
     void set_material_id(t_model_material_id material_id)
-        %code%{ THIS->material_id = material_id; %};
+        %code%{ THIS->material_id(material_id); %};
+    Ref<ModelMaterial> material();
     
     Ref<TriangleMesh> mesh()
         %code%{ RETVAL = &THIS->mesh; %};
diff --git a/xs/xsp/Point.xsp b/xs/xsp/Point.xsp
index 6fd39eac..4335c6eb 100644
--- a/xs/xsp/Point.xsp
+++ b/xs/xsp/Point.xsp
@@ -64,6 +64,7 @@ Point::coincides_with(point_sv)
         %code{% RETVAL = THIS->x; %};
     double y()
         %code{% RETVAL = THIS->y; %};
+    void translate(double x, double y);
 };
 
 %name{Slic3r::Pointf3} class Pointf3 {