A US appeals court said on Friday that Tesla Inc CEO Elon Musk violated federal labor laws when he tweeted that workers at the electric car maker would lose stock options if they joined a union, Reuters reported.

The court upheld a decision by the US National Labor Relations Board, which argued that the 2018 tweet was an illegal threat that could prevent unionization, and ordered Musk to remove it.

Amid an initiative by the United Auto Workers union at Tesla’s Fremont, California plant, Musk tweeted: “There is nothing stopping the Tesla team at our plant from voting for a union… But why pay union dues and give up stock options for nothing? “.

Tesla did not immediately respond to a request for comment.

In Friday’s case, Tesla argued that the tweet about unionization was not a threat, but merely reflected the fact that unionized workers at other auto companies were not granted stock options.

But a three-judge panel of the Fifth Circuit disagreed.

Musk’s prolific use of Twitter has also led to legal problems.

Musk bought Twitter for $44 billion last year.