diff --git a/src/xml/xml_api.cc b/src/xml/xml_api.cc
index e39b2a9c..ae9abe53 100644
--- a/src/xml/xml_api.cc
+++ b/src/xml/xml_api.cc
@@ -62,9 +62,8 @@ void GlobalModel::Clear() {
 
 
 // single instance of global model, protected with mutex
-GlobalModel themodel;
-std::mutex themutex;
-
+static GlobalModel themodel;
+static std::mutex themutex;
 
 //---------------------------------- Functions -----------------------------------------------------
 
diff --git a/src/xml/xml_util.h b/src/xml/xml_util.h
index 91e4bb09..5ba32aa3 100644
--- a/src/xml/xml_util.h
+++ b/src/xml/xml_util.h
@@ -76,7 +76,7 @@ class mjXSchema {
 
 // key(string) : value(int) map
 struct _mjMap {
-  std::string key;
+  const char* key;
   int value;
 };
 typedef struct _mjMap mjMap;