Using Cookie aware WebClient

I'm using this enhanced version of WebClient to login in a site:

public class CookieAwareWebClient : WebClient
{
        public CookieAwareWebClient()
        {
            CookieContainer = new CookieContainer();
        }
        public CookieContainer CookieContainer { get; private set; }

        protected override WebRequest GetWebRequest(Uri address)
        {
            var request = (HttpWebRequest)base.GetWebRequest(address);
            request.CookieContainer = CookieContainer;
            return request;
        }
}

And this way I send cookie to the site:

using (var client = new CookieAwareWebClient())
{
    var values = new NameValueCollection
    {
        { "username", "john" },
        { "password", "secret" },
    };
    client.UploadValues("http://example.com//dl27929", values);

    // If the previous call succeeded we now have a valid authentication cookie
    // so we could download the protected page
    string result = client.DownloadString("http://domain.loc/testpage.aspx");
}

But when I run my program and capture the traffic in Fiddler I get 302 status code. I tested the request in Fiddler this way and everything is OK and I get the status code of 200.
The request in the Fiddler:

GET http://example.com//dl27929 HTTP/1.1
Cookie: username=john; password=secret;
Host: domain.loc

And here is the request sending by the application:

POST http://example.com//dl27929 HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Host: www.domain.loc
Content-Length: 75
Expect: 100-continue
Connection: Keep-Alive

As you can see it doesn't send the cookie.
Any idea?