The United Auto Workers union has filed a complaint accusing Tesla chief executive Elon Musk of illegally threatening to take away benefits from workers who join the union.

The UAW, which is seeking to represent workers at Tesla’s facility in Fremont, California, filed the complaint with the National Labor Relations Board late on Wednesday.

Musk in a tweet on Monday said there was nothing stopping Tesla workers from joining a union, but “why pay union dues & give up stock options for nothing?”

The union says Musk violated the National Labor Relations Act, which prohibits employers from making threats or promises to workers to discourage them from joining unions.

Tesla in a statement said Musk’s comment simply recognized that other automakers whose workers are represented by the UAW do not provide stock options.

“UAW organizers have consistently dismissed the value of Tesla equity as part of our compensation package,” the company said.

In a separate tweet on Tuesday, Musk accused the UAW of driving General Motors and Chrysler to bankruptcy and losing “200,000+ jobs for people they were supposed to protect.”

He was apparently referring to effects of the crisis in the U.S. auto industry in 2008-2010.

UAW President Dennis Williams during a press briefing in Detroit on Thursday called Musk’s comments “ridiculous.”

“I don’t know what the hell Musk is up to,” Williams said. “Sometimes I scratch my head.”