WebRequest.Create problem

I'm trying to create a HttpWebRequest _webRequest = WebRequest.Create (_uri), where _uri a valid address. In this case, I get the error described here (http://msdn.microsoft.com/ru-ru/library/bw00b1dc (v = vs.95). aspx) as a NotSupportedException. This problem occurs only when the application is run from a browser (Opera, IE, etc.). In Unity (web player) everything works fine.