Property losing value even though declared Public

I've a file SiteMinder.CS in App_code where I set the UserID who has accessed the webpage

public class SiteMinder : IHttpModule
    {
        public string UserID { get; set; }

        public void Init(HttpApplication context)
        {
            context.PreRequestHandlerExecute += new EventHandler(Application_PreRequestHandler);
        }

        private void Application_PreRequestHandler(Object source, EventArgs e)
        {
            if (HttpContext.Current.Request.Headers != null)
            {
                NameValueCollection coll = HttpContext.Current.Request.Headers;
                UserID = coll["uid"]; // Doesn't have NULL value
            }
        }
    }

In another webpage UserDetails.aspx.cs file I'm trying to access this UserID but it is having NULL value.

public partial class UserDetails : System.Web.UI.Page
{

    protected string SessionUser { get; set; }
    protected void Page_Load(object sender, EventArgs e)
    {
        SiteMinder objSite = new SiteMinder();
        SessionUser = objSite.UserID;//Returns NULL
    }
}

All these are under same namespace. Please let me know where I'm wrong here.