Apple is offering a free battery replacements on some of its iPhone 6s models after discovering a defect that causes the phones to randomly shut down.

The company said Sunday that it has started the free repair program to address the fault in a “small number” of its 6s devices. “Apple has determined that a very small number of iPhone 6s devices may unexpectedly shut down” Apple said in a statement.

Apple said the defect only affects devices “within a limited serial number range” that were made between September and October 2015, and it assured customers there is no safety issue.