Walmart said on Wednesday that a technical issue in mid-March resulted in the retailer overcharging customers at some stores in the United States.

Bloomberg News, which first reported the incident, said Walmart suffered a technical glitch on March 19 that prevented price data from flowing to self-checkout kiosks at 1,600 stores.

The issue lasted for days and resulted in the company overcharging customers for thousands of items including food, clothes and appliances, the report said.

A spokesperson for Walmart, which operates more than 5,000 stores in the United States, said in an email that once it discovered the problem it focused on affected customers.

“We’ve made it a priority to refund customers who were overcharged, and we did not take action on the undercharges with our customers,” the spokesperson, Mischa Dunton, said.

Walmart did not respond to Reuters’ questions on the number of products impacted or the amount of dollars that shoppers overpaid. It also did not disclose how many stores were impacted.

In January, Walmart agreed to pay $45 million to settle a class-action lawsuit in Florida alleging it overcharged shoppers for bags of citrus fruit and some meat, poultry and seafood products sold by weight. The company denied any wrongdoing.

Since the pandemic, Walmart has invested billions of dollars in upgrading its stores and the underlying technology that powers thousands of its retail operations. In October last year, it said it would invest $9 billion in U.S. stores including adding more self-checkout options.

It has not always gone smoothly. In February, the Wall Street Journal reported Walmart suffered an outage where thousands of registers were not able to process transactions, while Bloomberg reported on Wednesday the company also endured a glitch with its photo and vision centers earlier this year.