WebClient and "Security Error" exception

Hello,

I tried to use the WebClient class, but I have a "Security Error" exception.
My code :

WebClient webClient = new WebClient();

webClient.DownloadStringCompleted += new DownloadStringCompletedEventHandler(webClient_DownloadStringCompleted);
webClient.DownloadStringAsync(new Uri(url));
 
And the exception occurred at the DownloadStringAsync method.

Thanks.
 
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