diff --git a/modules/sync/syncs/httpsynchronization.h b/modules/sync/syncs/httpsynchronization.h
index b5d17d71cb..75d10f8f2b 100644
--- a/modules/sync/syncs/httpsynchronization.h
+++ b/modules/sync/syncs/httpsynchronization.h
@@ -28,6 +28,7 @@
 #include <openspace/util/resourcesynchronization.h>
 
 #include <thread>
+#include <optional>
 #include <vector>
 
 namespace openspace {