SyndicationClient.RetrieveFeedAsync returning error

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var client = new SyndicationClient();
client.SetRequestHeader("accept", "text/html, application/xhtml+xml, */*");
client.SetRequestHeader("user-agent", "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)");