diff --git a/sol/sol_context.hpp b/sol/sol_context.hpp
index a857c7f..a071892 100644
--- a/sol/sol_context.hpp
+++ b/sol/sol_context.hpp
@@ -5,6 +5,7 @@
 
 #include <boost/asio/io_context.hpp>
 #include <boost/asio/steady_timer.hpp>
+#include <cstddef>
 
 namespace sol
 {