diff --git a/include/libtorrent/config.hpp b/include/libtorrent/config.hpp
index b6d30f7cb..09e36805e 100644
--- a/include/libtorrent/config.hpp
+++ b/include/libtorrent/config.hpp
@@ -33,6 +33,8 @@ POSSIBILITY OF SUCH DAMAGE.
 #ifndef TORRENT_CONFIG_HPP_INCLUDED
 #define TORRENT_CONFIG_HPP_INCLUDED
 
+#include <cstddef>
+
 #include "libtorrent/aux_/disable_warnings_push.hpp"
 
 #define _FILE_OFFSET_BITS 64