ViewState is not holding Password on postback

 i've tried using the Viewstate to hold the password of the user if the page posts back. but it is not working properly. Here is the LOC.
    protected string TypedPassword
    {
        get
        {
            if (ViewState["TypedPassword"] != null)
            {
                return Convert.ToString(ViewState["TypedPassword"]);
            }
            return null;
        }
        set
        {< ...