diff --git a/Phys/FunctorCore/include/Functors/Optional.h b/Phys/FunctorCore/include/Functors/Optional.h
index 4f6fb0219bc1db8d2b9a263419c6eef34b43c943..c073bc3985ea0c55fb0356a169447462a2cb04f1 100644
--- a/Phys/FunctorCore/include/Functors/Optional.h
+++ b/Phys/FunctorCore/include/Functors/Optional.h
@@ -26,6 +26,8 @@ namespace Functors {
    */
   template <typename T>
   struct Optional {
+    using value_type = T;
+
     Optional() = default;
     Optional( std::nullopt_t /*unused*/ ){};
     template <typename U = T,