diff --git a/websocket-sharp/Net/HttpListenerRequest.cs b/websocket-sharp/Net/HttpListenerRequest.cs
index 006882c5..b6e58f08 100644
--- a/websocket-sharp/Net/HttpListenerRequest.cs
+++ b/websocket-sharp/Net/HttpListenerRequest.cs
@@ -891,8 +891,7 @@ namespace WebSocketSharp.Net
     /// </exception>
     public X509Certificate2 GetClientCertificate ()
     {
-      // TODO: Not implemented.
-      throw new NotImplementedException ();
+      throw new NotSupportedException ();
     }
 
     /// <summary>