diff --git a/source/mijin/container/optional.hpp b/source/mijin/container/optional.hpp
index fb48971..6d49780 100644
--- a/source/mijin/container/optional.hpp
+++ b/source/mijin/container/optional.hpp
@@ -5,6 +5,7 @@
 #define MIJIN_CONTAINER_OPTIONAL_HPP_INCLUDED 1
 
 #include <cstdint>
+#include <functional>
 #include <utility>
 #include "../debug/assert.hpp"
 #include "../util/concepts.hpp"