diff --git a/src/operation/iCTS/api/CTSAPI.hh b/src/operation/iCTS/api/CTSAPI.hh
index 86ff3565..bef443a0 100644
--- a/src/operation/iCTS/api/CTSAPI.hh
+++ b/src/operation/iCTS/api/CTSAPI.hh
@@ -27,6 +27,7 @@
 #include <string_view>
 #include <type_traits>
 #include <vector>
+#include <optional>
 
 #include "../../../database/interaction/ids.hpp"