Apple will fix any iPhone 6s smartphone suffering from what the company says is an uncommon battery problem.

The tech giant AAPL will replace the battery on any iPhone 6s that shuts down unexpectedly, the company confirmed in a Support post on its website on Sunday. The replacement will be free, but limited to iPhone 6s units purchased between September and October 2015 that fall within a certain, but unidentified, serial number range.

The announcement comes just days after iPhone 6s owners in China were complaining that their devices were shutting down when they still had battery life left. Soon after, the Chinese consumer-protection agency, China Consumers Association, came to their defense and called on Apple to immediately address the problem.

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“In view that Apple iPhone 6 and iPhone 6s series cellphones in China have a considerable number of users, and the number of people who’ve reported this problem is rather many, China Consumer Association has already made a query with Apple,” the agency said in an open letter on Tuesday.

Soon after, Apple responded, saying that it had found the problem. Apple said last week that it was only affecting a small number of devices and wouldn’t cause a safety problem for users. Apple’s announcement over the weekend was confirmation that the company would address the problem.

On its support page, Apple says it will replace batteries at one of its retail stores or via an Authorized Service Provider. Wireless carriers will not participate in the replacement program. Apple also cautioned users to get their iPhones fixed before they tried to have their batteries replaced.

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“If your iPhone 6s has any damage such as a cracked screen which impairs the replacement of the battery, that issue will need to be resolved prior to the battery replacement. In some cases, there may be a cost associated with the repair,” Apple says.