New York (CNN) Tesla violated labor laws by threatening employees when they attempted to unionize, a California labor judge ruled late Friday.

The company must now hold a meeting to advise workers of their rights, and CEO Elon Musk is required to attend.

Administrative Law Judge Amita Baman Tracy ruled Friday that Tesla had violated the National Labor Relations Act multiple times in 2017 and 2018.

The violations included a Musk tweet in May 2018 saying, "Nothing stopping Tesla team at our car plant from voting union. Could do so tmrw if they wanted. But why pay union dues & give up stock options for nothing?"

The judge said the tweet amounted to "threatening employees" they would lose their stock options if they voted to unionize.

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