diff --git a/include/boost/intrusive/pack_options.hpp b/include/boost/intrusive/pack_options.hpp
index 3dc53a9..0ae349d 100644
--- a/include/boost/intrusive/pack_options.hpp
+++ b/include/boost/intrusive/pack_options.hpp
@@ -19,11 +19,11 @@
 #  pragma once
 #endif
 
+#include <cstddef>
+
 namespace boost {
 namespace intrusive {
 
-#include <cstddef>
-
 #ifndef BOOST_INTRUSIVE_DOXYGEN_INVOKED
 
 #if !defined(BOOST_INTRUSIVE_VARIADIC_TEMPLATES)