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Under a federal law known as the CARD Act, banks have been required since 2011 to cut back on penalty interest rates after cardholders resume timely payments. The bank said it found it erred in how it calculated some of those reductions.

While we believed our methodology was sound, a periodic internal review identified potential flaws in the methodology used to reevaluate interest rates on some credit card accounts,” Citigroup spokeswoman Liz Fogarty said in a statement.

“While we have found no evidence of employee misconduct, we should have identified these issues sooner,” the statement added.

Citigroup disclosed the problem generally in an annual securities filing early in the day and provided more details later.

Citigroup was the third biggest U.S. card lender in 2016, according to the Nilson Report.