Thousands of Chicago police officers received an letter from their pension fund this week that told them thanks to a payroll error spurred by the officers’ latest contract, approximately 3,000 are required to cut a check to their pension fund, plus interest.

The Fraternal Order of Police, Lodge 7, which represents most rank-and-file Chicago officers, said it planned on filing a grievance over the error so that the city would have to pay that interest charge instead of workers.

The flub is hitting Tier 2 members of the Policemen’s Annuity and Benefit Fund of Chicago, those who started working for the CPD on or after Jan. 1, 2011. Those members make up roughly half of the more than 12,000 active members of the fund. Sworn officers contribute 9% of their salary to their pension, which is automatically withdrawn from paychecks, the Chicago Tribune reports.

The union says the charge for some members is as low as about $80, and for others as high as $1,300.

“I know nobody likes to get a bill, and it should have never happened,” FOP President John Catanzara said in a video posted to the union’s YouTube page Monday. He blamed “incompetence” in CPD’s Finance Department and chastised the pension fund for not bringing the issue to the members’ attention earlier