Object reference not set to an instance of an object?? help

Im currently trying to write to an xml file on a button click event. So far i have this code.
protected void Button1_Click(object sender, EventArgs e)
    {
        XmlDocument doc = new XmlDocument();
        try
        {
            doc.Load(HttpContext.Current.Server.MapPath("~/App_Data/blog.xml"));
        }
        catch (Exception err)
        {
            //Error Message
            Response.Write("error: " + err.Message);
        }
       ...