If, in an action against a buyer of land for a broker's commissions, the plaintiff's evidence leaves it doubtful whether, while acting for defendant, he also was employed by A., the owner of the land, to sell it, or simply had an option on the property at the price named, the defendant is entitled to have the jury instructed that, "if the plaintiff acted as agent for A., without disclosing the fact that he was such agent to the defendant, he can not recover a commission from the defendant." Carpenter v. Fisher, 175 Mass. 9 55 N. E. 479. See also See. 314.