National Labor Relations Board found on Thursday that Elon Musk unlawfully threatened employees with loss of stock options if they chose to be represented by the United Auto Workers union with a tweet in 2018.

Tesla CEO threatened employees, preventing them from unionizing or supporting a labor union with loss of benefits.

The federal agency, NLRB ordered Musk to delete his tweet.

Musk had tweeted on May 20, 2018: “Nothing stopping Tesla team at our car plant from voting union. Could do so tmrw if they wanted. But why pay union dues and give up stock options for nothing? Our safety record is 2X better than when plant was UAW & everybody already gets healthcare.”

The agency also ordered Tesla to a notice addressing unfair labor practices at its Fremont, California, plant, and post a notice dealing with the tweet at all its facilities nationwide.

United Auto Workers Union has been trying to organize the 10,000-worker Fremont plant, called the order a great victory for workers who have the courage to stand up to companies like Tesla.