CHICAGO (AP) — Illinois officials say at least one non-U.S. citizen voted after an error in the state’s automatic vote registration system mistakenly registered over 500 people who said they weren’t citizens.

Secretary of State Jesse White’s office released the information Wednesday during a hearing in Springfield. On the same day, State Board of Election officials revised their numbers, saying 16 people of 545 registered voters cast ballots instead of 15. Nine voters are likely U.S. citizens since they have long voter histories, leaving six people in question.

White says he takes responsibility for the error and put his staff on notice.