Where the answer is a general denial, the issue presented by the pleading is the truth of the allegations of the petition; under such an issue affirmative proof in favor of the defendant cannot be received, and an instruction submitting such proof to the jury is erroneous; hence, evidence offered by defendant that he sold the premises to other parties before the sale by plaintiff is inadmissible. Griffith v. Woolworth, 44 N. W. 1137, 28 Neb. 715.