The error assigned is sufficient to maintain this writ of error. The mistake of the counsel for the defendant in error arises from not considering the distinction between the proper form of bringing the parties before the court and the legal liabilities which may properly be held to attach to an infant upon such trial when he is duly before the court. In the latter stage of the case it might constitute no defence that the party was a minor, if the cause of action was one to which *180infancy constituted no legal defence. But an infant cannot, in a civil suit, be properly defaulted, or have a judgment rendered against him for any cause of action, without having a legally appointed guardian, who may defend the suit in his behalf. If he has none under a probate appointment, or other authorized source, a guardian ad litem must be appointed, and the duty devolves upon the plaintiff in the action to have such appointment made, if no motion to that effect proceeds from the other side. Knapp v. Crosby, 1 Mass. 479. Valier v. Hart, 11 Mass. 300. Crockett v. Drew, 5 Gray, 399. We have considered this question as if raised by the plea in nullo est erratum, as it must be substantially taken to be, though not formally, and upon which view the case was argued. Judgment reversed.