diff --git a/include/kwk/concepts/values.hpp b/include/kwk/concepts/values.hpp
index f73da7d6..07ea8193 100644
--- a/include/kwk/concepts/values.hpp
+++ b/include/kwk/concepts/values.hpp
@@ -7,6 +7,8 @@
 //==================================================================================================
 #pragma once
 
+#include <cstddef>
+
 namespace kwk::concepts
 {
   template<typename T>