Citigroup Inc. mistakenly sent e-mails to some active Costco Wholesale Corp. members telling them their wholesale club membership had lapsed and that their cards would be canceled, bringing a fresh headache to the new partnership.

The e-mails, sent Friday, were meant for Costco customers who hadn’t renewed their memberships. The bank promptly alerted affected customers that their accounts remain active, and just a “small portion” of Costco cardholders received the errant communications, Jennifer Bombardier, a Citigroup spokeswoman, said Monday in a statement.

“We apologize for any concern or inconvenience this may have caused, and please be assured your account has not been compromised,” the New York-based bank said in a follow-up e-mail to cardholders. In a Facebook posting, the retailer’s customer service representatives described it as “a systematic error” and wrote that all affected accounts have been restored.

Citigroup struggled with the rollout of its Costco Anywhere Visa cards after taking over a portfolio of about 11 million accounts from American Express Co. in June, and Chief Executive Officer Mike Corbat said last month that the bank has deployed “a lot of resources” to ease a deluge of complaints. Costco customers flooded the retailer’s Facebook page in the days after the debut with hundreds of posts about lengthy hold times and trouble activating accounts.

Citigroup, the world’s biggest credit-card lender, ranked second-to-last in a J.D. Power credit-card customer satisfaction survey this year for the fourth consecutive year. Discover Financial Services ranked first for the second year in a row, narrowly edging out Amex.

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