diff --git a/cpp/mcap/include/mcap/types.hpp b/cpp/mcap/include/mcap/types.hpp
index f88e9a23e9..e78e410d61 100644
--- a/cpp/mcap/include/mcap/types.hpp
+++ b/cpp/mcap/include/mcap/types.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "errors.hpp"
+#include <cstddef>
 #include <functional>
 #include <limits>
 #include <memory>