Awash in new features like a 64-bit processor and gold, silver or “space gray” housing, the iPhone 5s still manages to eek out better battery life than its predecessors. Well, that’s supposed to be the case, but according to Apple (via the New York Times‘ Bits blog), a small number of the company’s new flagship phones have a manufacturing imperfection that’s causing the battery to bleed power much faster than expected. “We recently discovered a manufacturing issue affecting a very limited number of iPhone 5S devices that could cause the battery to take longer to charge or result in reduced battery life,” said Apple spokesperson Teresa Brewer in a statement. “We are reaching out to customers with affected phones and will provide them with a replacement phone.” It doesn’t sound like a problem with the batteries themselves, though “manufacturing issue” could refer to the phone or the batteries — it’s impossible to know without clarification from Apple. It’s also not clear how many phones a “very limited number” refers to. Apple reported first-weekend sales of around nine million smartphones, comprising both the iPhone 5s and 5c, when both models launched in September. And while Apple doesn’t specify this, that it’s “reaching out” to customers with affected phones implies it knows who has one and who doesn’t, possibly because it’s been able to trace the issue (whatever it is) to a particular manufacturing run.