diff --git a/src/tupleref.hpp b/src/tupleref.hpp
index 18bd7f4..3954d46 100644
--- a/src/tupleref.hpp
+++ b/src/tupleref.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cstddef>
 #include <tuple>
 #include <utility>