Rule 94 requires that "in pleading to a preceding pleading, a party shall set forth affirmatively . . . estoppel, . . . waiver, and any other matter constituting an avoidance or affirmative defense." TEX. R. CIV. P. 94. This rule is not limited to defendants but applies to all parties. See: Simmons v. Compania Financiera Libano, S.A., 830 S.W.2d 789, 792 (Tex. App.-Houston [1st Dist.] 1992, writ denied); Schachar v. Northern Assurance Co. of Am., 786 S.W.2d 766, 768 (Tex. App.-Dallas 1990, writ denied). A properly pleaded affirmative defense, supported by uncontroverted summary judgment evidence, may serve as the basis for a summary judgment. Roark v. Stallworth Oil & Gas, Inc., 813 S.W.2d 492, 494 (Tex. 1991). An unpleaded affirmative defense may also serve as the basis for a summary judgment when it is raised in the summary judgment motion, and the opposing party does not object to the lack of a rule 94 pleading. See id. Estoppel protects those who are misled, when one party knowingly induces another party to act or forebear to act under the induced mistaken belief. See Schroeder v. Texas Iron Works, Inc., 813 S.W.2d 483, 489 (Tex. 1991).