Mr. Chief Justice Moore,
after stating the facts, delivered the opinion of the court.
It is contended by plaintiff’s counsel that the court erred in giving, over their objection and exception, the following instruction: “Where two parties by mutual and unqualified agreement cause a boundary line to be established between their tracts, and build their fences upon such boundary line, and afterwards dispose of the lands owned by them respectively, with the division fence so marked upon the ground, and the purchaser of either of said parties buys, relying upon the line so established, the other party is estopped from denying that such line is the true boundary line; and if you believe from the evidence in this case that, at the time McDonald purchased this land from Hutchinson, the boundary line between the land purchased by him and the southeast quarter of said section 28 was marked on the ground by the fence where it now is, and McDonald bought relying upon it as £he true line, then I instruct you that the plaintiff is estopped from claiming that the line is other than the one so agreed to by the defendant, — if you find from the evidence such was made, your verdict must be for the defendant. ’ ’
1. The testimony given at the trial is incorporated in the bill of exceptions, and shows that in 1894 defendant purchased from Hutchinson the southwest quarter of said section 28, at which time the fence rebuilt by Walker and Hutchinson stood on the line as surveyed in 1885, and it is insisted by defendant’s counsel that their client most assuredly bought his land relying upon said fence as the true line, and did not know of any one claiming any land in the south half of said section lying west thereof; and it is argued that, as plaintiff and defendant are the parties mentioned in the answer as having acquiesced in the line so surveyed, as being the true boundary, the instruction complained of is within the issues, and that in any event it was not necessary for the jury to have believed that McDonald bought relying upon the fence as the true boundary. If a verdict is to be upheld because it was unnecessary for the'jury to believe all the instructions given them, *260thereby burdening them with the duty of determining not only the facts but also the law, it would irresistibly follow that, if a part of the court’s charge, notwithstanding its inconsistency or that it was without the issue, could be segregated, sufficient to support the verdict, no judgment would ever be reversed. Such is not the law, the rule being that, where an instruction outside the issues is given, it is impossible to say that the jury may not have observed and obeyed it and placed their verdict thereon: Morrison v. McAtee, 23 Or. 530 (32 Pac. 400). The rule is well settled in this state that an instruction outside the issues is erroneous, and constitutes reversible error: Marx v. Schwartz, 14 Or. 177 (12 Pac. 253); Woodward v. Oregon Ry. & Nav. Co. 18 Or. 289 (22 Pac. 1076); Coos Bay Nav. Co. v. Siglin, 26 Or. 387 (38 Pac. 192); Pearson v. Dryden, 28 Or. 350 (43 Pac. 166); Hughes v. McCullough, 39 Or. 372 (65 Pac. 85).
2. It is nowhere alleged that defendant purchased the southwest quarter of section 28 relying upon the location of the eastern boundary thereof as evidenced by the fence erected by Walker and Hutchinson in 1885, and standing at the time of his purchase on the line as then surveyed. The facts so assumed in the instruction complained of constitute an estop el in pais, and the rule is that, if a party has an opportunity to plead such an estoppel, he must do so to talie advantage thereof: Rugh v. Ottenheimer, 6 Or. 231 (25 Am. Rep. 513); Remillard v. Prescott, 8 Or. 37; Bruce v. Phoenix Ins. Co. 24 Or. 486 (34 Pac. 16); Bays v. Trulson, 25 Or. 109 (35 Pac. 26); Nickum v. Burckhardt, 30 Or. 464 (47 Pac. 788, 48 Pac. 474, 60 Am. St. Rep. 822). The defendant, prior to the trial, must have known the facts constituting the alleged estoppel, and therefore had an opportunity to plead them, but, not having done so, the instruction quoted is without the issues and erroneous.
It follows from these considerations that the judgment is rer versed and the cause remanded for a new trial. Reversed.