diff --git a/message_parsers.hpp b/message_parsers.hpp
index 489eb23..5738c43 100644
--- a/message_parsers.hpp
+++ b/message_parsers.hpp
@@ -3,6 +3,8 @@
 #include "message.hpp"
 #include "session.hpp"
 
+#include <cstddef>
+
 namespace message
 {