diff --git a/python/google/protobuf/pyext/message.cc b/python/google/protobuf/pyext/message.cc
index 3ff31e2..31d4003 100644
--- a/python/google/protobuf/pyext/message.cc
+++ b/python/google/protobuf/pyext/message.cc
@@ -121,11 +121,7 @@ namespace message_meta {
 namespace {
 // Copied over from internal 'google/protobuf/stubs/strutil.h'.
 inline void LowerString(std::string* s) {
-  std::string::iterator end = s->end();
-  for (std::string::iterator i = s->begin(); i != end; ++i) {
-    // tolower() changes based on locale.  We don't want this!
-    if ('A' <= *i && *i <= 'Z') *i += 'a' - 'A';
-  }
+  s->to_lower();
 }
 }  // namespace