This post has been updated and corrected.

Apple’s new watch is supposed to be its most personal device to date. It can measure your heart rate and lets you share it with others. But it seems it may not be quite as personal if you have tattoos on your forearms or darker skin.

In a post that first appeared on Reddit on April 28, a few early-adopter Apple Watch owners complained that they seemed to be having issues receiving notifications on their watches. These watch wearers had tattoos covering the area where a watch would sit on their wrists.

The Apple Watch will lock and require a passcode to use when it doesn’t detect skin contact and tattoos may be fooling the watches into thinking there’s no skin contact when there is. Some in the Reddit thread suggested the issue is a result of the sensor technology that Apple is using to check for skin contact and measure heart rates.

Steven LeBoeuf, a mechanical engineer scientist and the co-founder of Valencell, a company that supplies biometric sensors to wearable companies such as Jabra and Scosche, tells Quartz that Reddit’s theory is correct. Apple, like many wearable manufacturers, uses sensors that beam green light toward the skin. It penetrates through the first few layers of skin and measures the rate of bloodflow in the capillaries sitting below the surface. Green light, however, is absorbed by the ink used in most tattoos.

“Green light is a problem for anything dark, especially for tattoos,” LeBoeuf says.

While those with tattooed wrists may experience some issues, there’s also the possibility that the watch will not work as intended for a much larger group of potential watch buyers: those with darker skin. LeBoeuf says that green light is more likely to be absorbed by the skin of people with higher melanin content. Even if the sensors work when a person is sitting down, the darker their skin is, the less likely the sensors are to capture data when the person is moving. “The signal to noise ratio will be much lower for people having higher melanin content,” LeBoeuf says.

The solution to the problem may lie in changing the color of light used in the next Apple Watch, but most colors have issues. Ultraviolet light won’t penetrate through skin, and redder sources of light will penetrate too far, down to the bone. “The longer the wavelength, the deeper it will penetrate into the body,” he said. LeBoeuf said there are ways to get around the issue, including using a mixture of green and yellow lights, but no system will work perfectly.

Apple did not immediately respond to comment on whether the company is aware of this issue, or whether this came up in testing. Apple’s watch marketing displays individuals with a variety of skin tones, but LeBoeuf suggests Apple’s previously secret testing facility for the watch seems to have been less than diverse. ”There probably wasn’t tests on a broad enough sample of people with tattoos,” LeBoeuf said. (Quartz tested the Apple Watch’s heart rate monitor on an employee with tattoos on their arm, and out of three attempts, the watch only picked up their heart rate once.)

Rival wearable device company Basis—whose wearable uses a similar technology to measure heart rate—has a support query suggesting that tattoos could affect a device’s ability to pick up a heart rate. (Apple’s site explains what can affect its watch’s ability to detect a heart rate, but it doesn’t mention tattoos or skin color.)

Corrected April 30, 2014 (11:32am EST): The initial headline on this story has been changed to reflect that only people with tattoos have documented problems with the Apple Watch. The article only suggests the possibility that people with darker skin color might have similar problems. It was changed twice to convey this accurately.

Updated April 30, 2014 (11:05am EST): The comment by Steven LeBoeuf about signal to noise ratio has been expanded for clarity.