In an updated support document, Apple has acknowledged what some new Apple Watch owners quickly discovered over the last week: the Watch may not work properly for those with tattoos. The Watch's heart-rate monitor works by shining infrared or green light through the skin, but tattoos can interfere with that and prevent the Watch from getting a reading. "Permanent or temporary changes to your skin, such as some tattoos, can also impact heart-rate sensor performance," Apple writes. "The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings."
Apple's description is vague about what type of tattoos will and won't pose a problem, perhaps because it doesn't precisely know. What's been widely suggested is that darker tattoos are the ones causing issues, preventing too much of the Watch's light from getting through. This isn't just a problem related to reading heart rates. Apple also uses those sensors to detect when the Watch has been removed from a person's wrist — if the Watch doesn't think it's being worn, it won't receive notifications; it can also cause the Watch to lock if a passcode is enabled. There are work arounds for anyone running into this problem, but they remove some of the conveniences and security that are supposed to be built in.
Read next: Read our Apple Watch review.