HIGHWOOD, Ill. (CBS) – A north suburban alderman was resigning Thursday, after learning he couldn’t continue holding his position because he’s not a U.S. citizen.

WBBM Newsradio’s Bernie Tafoya reports Highwood Ald. Harvey Knapp was appointed to the city council five months ago to fill a vacancy. At the time, neither he nor Highwood’s mayor realized state law requires municipal officials to be citizens.

In December, Knapp began filling out paperwork to run for a full term in April, but learned he wasn’t eligible.

“We all make mistakes, I guess, as we go along, but you deal with them and you move forward,” he said. “That’s about all I can do. I feel fortunate to have been able to serve for the period of time that I did, and I hope that at some point I can get back and do something.”

Knapp has been in the United States for 23 years, and considers himself to be an American, but he’s a British native and has not received U.S. citizenship.

Knapp said he’s always thought about becoming a citizen, but that “life got in the way,” and citizenship got pushed to the back burner.

The Highwood city council is expected to accept Knapp’s resignation on Thursday. He said he’ll now apply for citizenship.