Elon Musk joined Trump on Capitol Hill for a meeting with Republicans—and the president-elect used the opportunity to ridicule the richest man in the world.

While we don’t know the substance of what was discussed, Musk’s presence is a concerning preview of Trump’s often reckless approach to diplomacy — as well as his disregard for the line between private interests and public policy. There are a lot of reasons why Musk should be the last person hopping on that call.