Elon Musk’s social media company, X (formerly known as Twitter), broke the law when it fired one of its employees for “exercising their right to protected concerted activity” following X’s return-to-office mandate, according to a complaint from the National Labor Relations Board.

The complaint alleges that a high-ranking X official told workers, “If you can physically make it to an office and you don’t show up, resignation accepted.” One employee reached out to coworkers on Slack and encouraged them not to resign but instead wait to get fired. The employee was illegally terminated for their actions, according to the complaint.

The NLRB protects workers’ legal right to engage in “concerted activity,” which is when two or more coworkers band together to address workplace issues.