diff --git a/src/lib/base/Event.h b/src/lib/base/Event.h
index 38a2cf11..7f4461c9 100644
--- a/src/lib/base/Event.h
+++ b/src/lib/base/Event.h
@@ -18,6 +18,7 @@
 
 #pragma once
 
+#include <cstddef>
 #include "common/basic_types.h"
 #include "common/stdmap.h"