diff --git a/include/tulz/observer/EternalObserverFactory.h b/include/tulz/observer/EternalObserverFactory.h
index 65c519e..321e29b 100644
--- a/include/tulz/observer/EternalObserverFactory.h
+++ b/include/tulz/observer/EternalObserverFactory.h
@@ -6,7 +6,7 @@
 
 namespace tulz {
 namespace detail {
-using EternalObserverFactory_t = decltype([]<typename ...Args>(EternalObserver<Args...>::Func f = {}) {
+using EternalObserverFactory_t = decltype([]<typename ...Args>(typename EternalObserver<Args...>::Func f = {}) {
     return std::make_unique<EternalObserver<Args...>>(std::move(f));
 });
 }