diff --git a/sol/sol_manager.hpp b/sol/sol_manager.hpp
index 6a5887b..74ce8e9 100644
--- a/sol/sol_manager.hpp
+++ b/sol/sol_manager.hpp
@@ -6,6 +6,7 @@
 
 #include <boost/asio/io_context.hpp>
 #include <boost/asio/local/stream_protocol.hpp>
+#include <cstddef>
 #include <map>
 #include <memory>
 #include <string>