diff --git a/main/Scene.cc b/main/Scene.cc
index 4373370..6fba3b6 100644
--- a/main/Scene.cc
+++ b/main/Scene.cc
@@ -13,7 +13,7 @@
 #include "PointLight.h"
 using namespace std;
 
-Scene::Scene(map<string, const char *> options,
+Scene::Scene(const map<string, const char *> & options,
              const char * filename)
 {
     m_width = 800;
@@ -27,7 +27,7 @@ Scene::Scene(map<string, const char *> options,
     load(filename);
 
     /* after loading the scene file, apply any command-line render options */
-    for (map<string, const char *>::iterator it = options.begin();
+    for (map<const string, const char *>::const_iterator it = options.begin();
          it != options.end();
          it++)
     {
diff --git a/main/Scene.h b/main/Scene.h
index a82f5c6..52dd719 100644
--- a/main/Scene.h
+++ b/main/Scene.h
@@ -26,7 +26,7 @@ class Scene
                 };
         };
 
-        Scene(std::map<std::string, const char *> options,
+        Scene(const std::map<std::string, const char *> & options,
               const char * filename);
         ~Scene();
         void render();