diff --git a/include/boost/core/pointer_traits.hpp b/include/boost/core/pointer_traits.hpp
index e0ebfb0..e66194d 100644
--- a/include/boost/core/pointer_traits.hpp
+++ b/include/boost/core/pointer_traits.hpp
@@ -13,6 +13,7 @@ Distributed under the Boost Software License, Version 1.0.
 #include <memory>
 #else
 #include <boost/core/addressof.hpp>
+#include <cstddef>
 #endif
 
 namespace boost {