The U.S. Supreme Court is raising doubts about Alaska’s $500-a-year limit on contributions to political candidates.

The justices are ordering a lower court to take a new look at the issue.

The court says in an unsigned opinion on Monday that federal judges who rejected a challenge to the contribution cap did not take into account a 2006 high court ruling.

The 2006 ruling invalidated low-dollar limits on political contributions in Vermont.

Justice Ruth Bader Ginsburg wrote a separate opinion expressing that Alaska’s reliance on the energy industry may make the state unusually vulnerable to political corruption and justify the low limits.