Appellant, Kathryn Fite, as the mother of C_D_F_, a minor child,1 appeals from a take-nothing summary judgment in favor of appellee, Barney Ray King, in Fite’s action to establish the paternity of the child. In her sole point of error, Fite contends that the doctrine of res judicata does not bar her action. We disagree. Accordingly, we affirm.
The present case is the second suit brought by Fite, in her capacity as mother of the child, against King to establish the child’s paternity. In the first action, brought in 1981, the trial court rendered summary judgment against Fite. The summary judgment proof in the present case contains Fite’s trial pleadings, the judgment, and the order overruling Fite’s motion for new trial in the 1981 action. Although Kings’s summary judgment motion and proof in the present case does not inform this Court of the basis upon which the trial court rendered a take-nothing judgment against Fite in the first action, both parties state that the trial court based its judgment on the one-year statute of limitations contained in section 13.01 of the Texas Family Code as it existed in 1981. Accordingly, we treat the 1981 judgment as a take-nothing judgment rendered solely upon the ground that Fite’s cause of action was barred by the one-year statute of limitations contained in section 13.01 as it existed in 1981. Finnigan v. Blanco County, 670 S.W.2d 313, 318 (Tex.App.—Austin 1984, no writ); TEX.R.CIV.P. 419. It is clear from King’s motion for summary judgment in this second action that the trial court’s take-nothing judgment in the present case is based on the doctrine of res judicata. Fite did not appeal from the 1981 judgment, and that judgment became final.
This appeal is considered only in light of the existing parties. Both actions against King were brought in Fite’s capacity as mother of the child; the child has not been represented in an individual capacity.2 We do not pass judgment on the child’s rights, as that issue is not before this court.
Fite argues that the doctrine of res judi-cata should not preclude her second suit for three reasons: (1) the Texas Family Code amendments allow a previously barred claim to be litigated; (2) the law barring the original action was declared to be unconstitutional; and (3) the original case was not decided on the merits. Fite’s appeal is based in part upon section 2 of the 1983 amendment to Family Code section 13.01, which provides:
SECTION 1. Section 13.01, Family Code, is amended to read as follows: Sec. 13.01. TIME LIMITATION OF SUIT. A suit to establish the parent-child relationship between a child who is not the legitimate child of a man and the child’s natural father by proof of paternity must be brought on or before the second anniversary of the day the child becomes an adult [the child is four years ©1¾ or the suit is barred.
SECTION 2. A cause of action that was barred before the effective date of this Act but would not have been barred by Section 13.01, Family Code, as amended by this Act, is not barred until the period of limitations provided by Section 13.01, Family Code, as amended by this Act, has expired.
Act of June 19, 1983, ch. 744, secs. 1, 2, 1983 Tex.Gen.Laws 4530-31.
In this action Fite seeks the same relief sought in the 1981 suit. Under the doctrine of res judicata, a final judgment ren*347dered against a claimant bars later relit-igation of the same cause of action. This is “technical” res judicata or “claim preclusion.” Gilbert v. Fireside Enterprises, Inc., 611 S.W.2d 869, 871 (Tex.Civ.App.—Dallas 1980, no writ). When a question of fact or law is put in issue and the court renders a final judgment on a ground of recovery or defense, there can be no subsequent suit on the same issues, whether the second suit is for the same, or a different cause of action. Hammonds v. Holmes, 559 S.W.2d 345, 346 (Tex.1977); Olivarez v. Broadway Hardware, Inc., 564 S.W.2d 195, 198 (Tex.Civ.App.—Corpus Christi 1978, writ ref'd n.r.e.).
We consider Fite’s argument that the doctrine of res judicata does not apply because the one-year period afforded by section 13.01 for establishing paternity was declared unconstitutional as it denies illegitimate children in Texas the equal protection of law. Mills v. Habluetzel, 456 U.S. 91, 102, 102 S.Ct. 1549, 1556, 71 L.Ed.2d 770 (1982). An unconstitutional act confers no right, imposes no duty, and affords no protection. Chicago, Indianapolis & Louisville Railway Co. v. Hackett, 228 U.S. 559, 566, 33 S.Ct. 581, 584, 57 L.Ed. 966 (1913); King v. King, 291 S.W. 645, 648 (Tex.Civ.App.—San Antonio 1927, writ dism’d w.o.j.). That, however, is not the issue in the present case. In the present case, Fite acquiesced in the application of the unconstitutional act and did not appeal. Indeed, Fite could have pursued the matter, as did the natural mother in Mills, and secured the same holding as did the natural mother in Mills. Instead, Fite stood by and allowed an adverse judgment to become final. The fact that a statute upon which a judgment is based is unconstitutional and the error correctable on appeal, does not make the judgment void ab initio so as to render it assailable as a void judgment in a collateral attack. See King, 291 S.W. at 648. We conclude, therefore, that the fact that the statute barring the original action was declared unconstitutional, after the judgment in the original action had become final, does not prevent application of the doctrine of res judicata in the present case. See Slater v. Blackwood, 15 Cal.3d 791, 126 Cal.Rptr. 225, 543 P.2d 593 (1975).
Fite asserts that the first paternity suit was not decided on the merits. Res judicata requires a “valid and final personal judgment on the merits of a plaintiff’s cause of action” in order to preclude relit-igation on the same cause of action. Hammonds, 559 S.W.2d at 346 (Tex.1977); Gilbert, 611 S.W.2d at 871. Fite ignores the fact that a proper affirmative defense may satisfy the requirements for the rendition of a valid and final judgment. Hammond, 559 S.W.2d at 346; Olivarez, 564 S.W.2d at 198. King’s affirmative defense of limitations was the basis of the first summary-judgment against Fite. This was a valid and final judgment on the merits of the cause of actions.
Finding no merit in Fite’s assertion that the doctrine of res judicata does not bar her action in the present case, we overrule Fite’s sole point of error. As this case is disposed of by the doctrine of res judicata, it is not necessary to address the constitutional challenge to section 2 of the 1983 amendment to section 13.01 of the Texas Family Code. City of San Antonio v. Schautteet, 706 S.W.2d 103 (Tex.1986).
Affirmed.
GUITTARD, C.J., and AKIN, STEPHENS, GUILLOT, HOWELL, SCALES, STEWART and HOLLINGSWORTH, JJ., join in this opinion. WHITHAM, J., files a concurring opinion. VANCE, DEVANY and McCLUNG, JJ., join in the concurring opinion.. We note that the minor child, C_D_ F_, has not been a party to either suit in any capacity. See TEX.R.CIV.P. 44 and 173.
. Fite did not bring the two suits as next friend or guardian ad litem for the minor child. A minor child cannot bring a cause of action on its own unless: (1) disability has been removed, Sax v. Votteler, 648 S.W.2d 661, 666-67 (Tex.1983); (2) it is represented by a "next friend”, see Safeway Stores Inc. v. Rutherford, 130 Tex. 465, 111 S.W.2d 688, 689 (1938); TEX.R.CIV.P. 44; or (3) it is represented by a guardian ad litum; see Pleasant Hills Children's Home of the Assemblies of God, Inc. v. Nida, 596 S.W.2d 947, 951 (Tex.Civ.App.—Fort Worth 1980, no writ); TEX.R.CIV.P. 173.