diff --git a/include/boost/none_t.hpp b/include/boost/none_t.hpp
index 9ed9409..6b3c833 100644
--- a/include/boost/none_t.hpp
+++ b/include/boost/none_t.hpp
@@ -13,6 +13,8 @@
 #ifndef BOOST_NONE_T_17SEP2003_HPP
 #define BOOST_NONE_T_17SEP2003_HPP
 
+#include <boost/config.hpp>
+
 namespace boost {
 
 #ifdef BOOST_OPTIONAL_USE_OLD_DEFINITION_OF_NONE
@@ -29,7 +31,7 @@ class none_t {};
 struct none_t
 {
   struct init_tag{};
-  explicit constexpr none_t(init_tag){} // to disable default constructor
+  explicit BOOST_CONSTEXPR none_t(init_tag){} // to disable default constructor
 };
 
 #endif // old implementation workarounds