SSRS2012 forms authentication cookie domain is missing

Switched SSRS2012 to forms authentication:

C:\Program Files\Microsoft SQL Server\MSRS11.MSSQLSERVER\Reporting Services\ReportServer\web.config

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The cookie name is correct in the actual cookie but the domain is empty / not set for some reason.  The empty cookie domain causes a subsequent error in ReportingServiceProxy when trying to add the auth cookie to the request.CookieContainer.

protected override WebRequest GetWebRequest(Uri uri)
 {
            HttpWebRequest request;
            request = (HttpWebRequest)HttpWebRequest.Create(uri);
            // Create a cookie jar to hold the request cookie
            CookieContainer cookieJar = new CookieContainer();
            request.CookieContainer = cookieJar;
            Cookie authCookie = AuthCookie;
            // if the client already has an SSRS auth cookie
            // place it in the request's cookie container
            if (authCookie != null)
            {
                request.CookieContainer.Add(authCookie);  //Exception thrown here because of empty cookie domain
            }
            request.Timeout = -1;

Why is the forms auth / ssrs cookie domain empty?  How do you resolve this?

thanks

scott