diff --git a/learn/stl/include/ustl/type_traits b/learn/stl/include/ustl/type_traits
index 722fbb39e..73906375d 100644
--- a/learn/stl/include/ustl/type_traits
+++ b/learn/stl/include/ustl/type_traits
@@ -10,6 +10,8 @@
 //
 #include <type_traits>
 
+#include <cstddef>
+
 namespace ustl {
 
 // Forward declaration of helpers: