diff --git a/ml.cpp b/ml.cpp
index 084ec26..1db1db1 100644
--- a/ml.cpp
+++ b/ml.cpp
@@ -1708,7 +1708,7 @@ bool MlEnvironment::has(std::string name) const {
 }
 
 // Get the value associated with this name in this scope
-MlValue MlEnvironment::get(std::string name) const {
+MlValue MlEnvironment::get(const std::string& name) const {
     // Meta operations
     if (name == "eval") return MlValue("eval", builtin::eval);
     if (name == "type") return MlValue("type", builtin::get_type_name);
@@ -1790,7 +1790,7 @@ MlValue MlEnvironment::get(std::string name) const {
 
     std::map<std::string, MlValue>::const_iterator itr = defs.find(name);
     if (itr != defs.end()) return itr->second;
-    else if (parent_scope != NULL) {
+    else if (parent_scope != nullptr) {
         itr = parent_scope->defs.find(name);
         if (itr != parent_scope->defs.end()) return itr->second;
         else return parent_scope->get(name);
diff --git a/ml.h b/ml.h
index e5d8036..10da210 100644
--- a/ml.h
+++ b/ml.h
@@ -28,7 +28,7 @@ public:
     // creating a lambda function.
     bool has(std::string name) const;
     // Get the value associated with this name in this scope
-    MlValue get(std::string name) const;
+    MlValue get(const std::string& name) const;
     // Set the value associated with this name in this scope
     void set(std::string name, MlValue value);