diff --git a/src/dotnetCampus.ApplicationStartupManager/StartupManagerBase.cs b/src/dotnetCampus.ApplicationStartupManager/StartupManagerBase.cs
index d906aaa..ec23e79 100644
--- a/src/dotnetCampus.ApplicationStartupManager/StartupManagerBase.cs
+++ b/src/dotnetCampus.ApplicationStartupManager/StartupManagerBase.cs
@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;