A plaintiff properly filed a complaint in federal district court under diversity jurisdiction against an electrician. According to the complaint, the electrician allegedly negligently performed electrical work in a building owned by the plaintiff, which resulted in a fire that caused $150,000 in damages to the building. The electrician served an answer to the complaint. Twelve days later, the electrician filed with the court and served, without leave of the court, a summons and third-party complaint on his partner. The electrician and his partner are citizens of different states. According to this complaint, if the electrician is liable to the plaintiff, the partner is liable to the electrician for half of the damages. The partner has moved to dismiss this claim against him. How should the court rule?