An appeals court on Tuesday sided with special counsel Robert Mueller in a legal dispute brought by Andrew Miller, a Roger Stone associate who had been subpoenaed in Mueller’s investigation.

Miller, in seeking to quash the subpoena, had argued that Mueller’s appointment was unlawful. An appeals court rejected that argument on Tuesday, while backing a lower court’s order holding Miller in contempt for resisting the subpoena.

A conservative legal group had used Miller’s subpoena as a vehicle to challenge Mueller’s authority. The group’s leaders were inspired by an op-ed by law professor Steven Calabresi, who has argued that Mueller’s appointment was unconstitutional under the Appointments Clause.

U.S. District Court of D.C. Chief Judge Beryl A. Howell ruled against those arguments last summer. Miller appealed her decision and a three-judge appeals court panel in in D.C. heard oral arguments on the dispute in November.

“The statutory and regulatory scheme demonstrate, contrary to Miller’s contention, that at the time of Special Counsel Mueller’s appointment, Acting Attorney General Rosenstein was the ‘Head of Department’ under the Appointments Clause as to the matter on which the Attorney General was recused,” the court wrote in its opinion Tuesday.

Miller’s lawyer Paul Kamenar has already vowed to take the case to the Supreme Court if he lost at the appellate level. The Supreme Court already has one other legal dispute dealing with a Mueller subpoena it is considering taking up — specifically a challenge to a subpoena issued for an unidentified foreign-owned corporation.

Stone, meanwhile, was indicted by a grand jury last month. After his arrest, Kamenar told reporters that the special counsel had not sought to drop the subpoena of his client, who served as an aide to Stone in 2016.

Read the opinion below: