diff --git a/websocket-sharp/Net/HttpListenerRequest.cs b/websocket-sharp/Net/HttpListenerRequest.cs
index e5f8ecf7..698ccc77 100644
--- a/websocket-sharp/Net/HttpListenerRequest.cs
+++ b/websocket-sharp/Net/HttpListenerRequest.cs
@@ -131,7 +131,7 @@ namespace WebSocketSharp.Net
     /// </value>
     public int ClientCertificateError {
       get {
-        return 0; // TODO: Always returns 0.
+        throw new NotSupportedException ();
       }
     }