(CBS SF) — It looks like Apple’s product development team doesn’t have arm tattoos, or at least didn’t seek out people who do in order to make sure Apple Watches function properly.

Reports are funneling in that tattooed wrists are blocking Apple Watch’s heart rate sensor which is needed to do things like use Apple Pay, receive notifications and place calls.

Reddit users first complained that when the Watch was strapped onto tattooed skin, the screen would lock up as if the person wasn’t wearing it. But when the Watch was moved to a non-tattooed spot of skin, it would function perfectly.

Apple Watch switches on when it senses your heartbeat from your wrist. The back of the Watch is a heart rate sensor that uses LED lights to detect the amount of blood flowing through your wrist at any given moment.

CNN Money reports the Watch works fine with dark-colored skin, scars and skin abrasions, which are translucent and allow light to pass through. But outside light struggles to seep through opaque tattoo ink.

Apple did not immediately respond to a request for comment.