A judge has ruled that Tesla violated the National Labor Relations Act in 2017 and 2018, Bloomberg reports.

In a tweet published on May 21, 2018, Tesla chief executive Elon Musk suggested that employees who chose to join a union would give up company-paid stock options. This was one of a number of incidents the judge concluded were in violation of the law.

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“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? Our safety record is 2X better than when plant was UAW & everybody already gets healthcare,” Musk tweeted.

The judge order calls for the electric car manufacturer to reinstate and back-pay a fired pro-union employee, and to revoke a warning issued to another union supporter. In addition, Tesla will be required to hold a meet at its assembly plant in Fremont, California, where Musk or an agent with the labor board will be required to read a notice to employees informing them that the electric automaker broke the law.

Tesla has previously denied any wrongdoing and is expected to challenge the National Labor Relations Board’s determination. The NLRB is unable to assess punitive damages or hold executives personally liable for violations and can only require companies reinstate and pay back wages to workers that were illegally fired.