diff --git a/xs/src/libslic3r/Model.cpp b/xs/src/libslic3r/Model.cpp
index 5243799ca..d3127f917 100644
--- a/xs/src/libslic3r/Model.cpp
+++ b/xs/src/libslic3r/Model.cpp
@@ -753,18 +753,20 @@ ModelObject::mirror(const Axis &axis)
 }
 
 void
-ModelObject::transform_by_instance(const ModelInstance &instance, bool dont_translate)
+ModelObject::transform_by_instance(ModelInstance instance, bool dont_translate)
 {
+    // We get instance by copy because we would alter it in the loop below,
+    // causing inconsistent values in subsequent instances.
     this->rotate(instance.rotation, Z);
     this->scale(instance.scaling_factor);
     if (!dont_translate)
         this->translate(instance.offset.x, instance.offset.y, 0);
     
-    for (ModelInstancePtrs::iterator i = this->instances.begin(); i != this->instances.end(); ++i) {
-        (*i)->rotation -= instance.rotation;
-        (*i)->scaling_factor /= instance.scaling_factor;
+    for (ModelInstance* i : this->instances) {
+        i->rotation -= instance.rotation;
+        i->scaling_factor /= instance.scaling_factor;
         if (!dont_translate)
-            (*i)->offset.translate(-instance.offset.x, -instance.offset.y);
+            i->offset.translate(-instance.offset.x, -instance.offset.y);
     }
     this->origin_translation = Pointf3(0,0,0);
     this->invalidate_bounding_box();
diff --git a/xs/src/libslic3r/Model.hpp b/xs/src/libslic3r/Model.hpp
index 36571ff5d..fed54b601 100644
--- a/xs/src/libslic3r/Model.hpp
+++ b/xs/src/libslic3r/Model.hpp
@@ -160,7 +160,7 @@ class ModelObject
     void scale_to_fit(const Sizef3 &size);
     void rotate(float angle, const Axis &axis);
     void mirror(const Axis &axis);
-    void transform_by_instance(const ModelInstance &instance, bool dont_translate = false);
+    void transform_by_instance(ModelInstance instance, bool dont_translate = false);
     size_t materials_count() const;
     size_t facets_count() const;
     bool needed_repair() const;