An affirmative defense to be proved must be pleaded. Kelly v. Stone, 94 Iowa, 316, 62 N. W. 842; Scott v. Dillon, 109 N. Y. S. 877, 58 Misc. 522; Mauser v. Hurdle, 140 P. 479, 27 Colo. App 567; Langston v. Hoyt, 194 P. 654, - Kan. Sup. - .