The court of appeals reversed, reasoning that because the OAG was enforcing the child support order on the mother's behalf as her assignee, it was subject to all affirmative defenses that could be asserted by one private party against another, including estoppel. 352 S.W.3d 48, 52, 55. It instructed the trial court to conduct a hearing on Scholer's estoppel defense. Id. at 55.