diff --git a/websocket-sharp/Net/HttpListenerRequest.cs b/websocket-sharp/Net/HttpListenerRequest.cs
index 90eb56bb..e5ad7d65 100644
--- a/websocket-sharp/Net/HttpListenerRequest.cs
+++ b/websocket-sharp/Net/HttpListenerRequest.cs
@@ -873,8 +873,7 @@ namespace WebSocketSharp.Net
     /// </exception>
     public X509Certificate2 EndGetClientCertificate (IAsyncResult asyncResult)
     {
-      // TODO: Not implemented.
-      throw new NotImplementedException ();
+      throw new NotSupportedException ();
     }
 
     /// <summary>