diff --git a/include/boost/mp11/set.hpp b/include/boost/mp11/set.hpp
index 3df0841..af18af8 100644
--- a/include/boost/mp11/set.hpp
+++ b/include/boost/mp11/set.hpp
@@ -13,6 +13,7 @@
 #include <boost/mp11/detail/mp_list.hpp>
 #include <boost/mp11/detail/mp_append.hpp>
 #include <boost/mp11/detail/mp_copy_if.hpp>
+#include <boost/mp11/detail/mp_fold.hpp>
 #include <boost/mp11/detail/mp_remove_if.hpp>
 #include <boost/mp11/detail/mp_is_list.hpp>
 #include <type_traits>