diff --git a/include/boost/smart_ptr/detail/sp_convertible.hpp b/include/boost/smart_ptr/detail/sp_convertible.hpp
index 31b2627..4bba9ed 100644
--- a/include/boost/smart_ptr/detail/sp_convertible.hpp
+++ b/include/boost/smart_ptr/detail/sp_convertible.hpp
@@ -16,6 +16,7 @@
 //  http://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/config.hpp>
+#include <cstddef>
 
 #if !defined( BOOST_SP_NO_SP_CONVERTIBLE ) && defined( BOOST_NO_SFINAE )
 # define BOOST_SP_NO_SP_CONVERTIBLE