diff --git a/websocket-sharp/Net/HttpListenerContext.cs b/websocket-sharp/Net/HttpListenerContext.cs
index 96040976..6488a978 100644
--- a/websocket-sharp/Net/HttpListenerContext.cs
+++ b/websocket-sharp/Net/HttpListenerContext.cs
@@ -246,6 +246,9 @@ namespace WebSocketSharp.Net
 
     internal void Unregister ()
     {
+      if (_listener == null)
+        return;
+
       _listener.UnregisterContext (this);
     }