diff --git a/include/boost/config/stdlib/msl.hpp b/include/boost/config/stdlib/msl.hpp
index 7e875922..4c6bad4a 100644
--- a/include/boost/config/stdlib/msl.hpp
+++ b/include/boost/config/stdlib/msl.hpp
@@ -35,7 +35,7 @@
 #  include <boost/config/posix_features.hpp>
 #endif
 
-#if defined(_MWMT) || defined(_MSL_THREADSAFE)
+#if defined(_MWMT) || _MSL_THREADSAFE
 #  define BOOST_HAS_THREADS
 #endif