HTTPContext ToString()Cannot implicitly convert type 'string' to 'bool'
===

This is probably a quick simple answer, but all i'm trying to do is verify some keys against a database, I keep getting string to bool error which to me makes no sense.

    if (HttpContext.Current.Session["UserName"].ToString() == "")
            {
                Server.Transfer("login.aspx");
            }

I just want to know how to fix this, not change the code all around.  Thanks