diff --git a/src/sexpr/Casting.hpp b/src/sexpr/Casting.hpp
index a13298f..5273cfc 100644
--- a/src/sexpr/Casting.hpp
+++ b/src/sexpr/Casting.hpp
@@ -5,6 +5,7 @@
 #include "SExpr.hpp"
 #include <functional>
 #include <memory>
+#include <optional>
 #include <sstream>
 
 namespace sexpr {