diff --git a/websocket-sharp/Net/HttpListenerPrefixCollection.cs b/websocket-sharp/Net/HttpListenerPrefixCollection.cs
index 4a725704..1e585194 100644
--- a/websocket-sharp/Net/HttpListenerPrefixCollection.cs
+++ b/websocket-sharp/Net/HttpListenerPrefixCollection.cs
@@ -190,10 +190,11 @@ namespace WebSocketSharp.Net
     /// </exception>
     public bool Contains (string uriPrefix)
     {
-      _listener.CheckDisposed ();
       if (uriPrefix == null)
         throw new ArgumentNullException ("uriPrefix");
 
+      _listener.CheckDisposed ();
+
       return _prefixes.Contains (uriPrefix);
     }