The full U.S. Court of Appeals for the Fifth Circuit on Thursday questioned whether to defer to the National Labor Relation Board’s conclusion that a 2018 tweet from Tesla CEO Elon Musk illegally threatened workers with the loss of benefits if they unionized.

A three-judge panel of the court had upheld the NLRB’s finding of an unfair labor practice that relied on the 1969 U.S. Supreme Court ruling NLRB v. Gissel Packing, which calls for courts to defer to the NLRB if the agency has substantial evidence an employer’s written statement about unionization is reasonably understood as a threat or coercive.