The complaint must allege either performance or a valid excuse for non-performance, and the proof must conform to the allegations of the petition. Daly v. Russ, 86 Cal. 114, 24 P. 867; Norman v. Reuther, 54 N. Y. S. 152, 25 Misc. 161; Tar-borough v. Creager (Tex. Civ. App. '03), 77 S. W. 645; Martin v. Fagan, 88 N. Y. S. 472, 95 App. Div. 154; Hoefling v. Ham-bleton, 84 Tex. 517, 19 S. W. 689.