Tattooed tech lovers, take note: The Apple Watch’s heart rate sensor may not work for people with tattoos covering their wrists.

According to iMore, watch owners with inked-up wrists have noted on Twitter and Reddit that they have had trouble getting the device to read their heartbeats. The problem is likely occurring because the watch’s sensors utilize LED lights to determine when blood flow is increasing on the wrist during each heartbeat.

Apple explains the process in a support article for the watch:

"Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate."

The presence of artificial ink on the skin can apparently interfere with this process, according to iMore, by reducing the Apple Watch sensor’s ability to detect changes in light. The tech news site tried to replicate some of the complaints using tattooed and non-tattooed wrists. In particular, tattoos that make use of dark colors could make the heart rate monitoring feature yield faulty results. Natural skin pigments don’t seem to cause the same issue, but other things like scars might.

Apple acknowledges in its support article that “various factors” could make it impossible for a small subset of users to get a heart rate reading, but the company doesn’t specify what those factors are. Potential buyers who sport ink should probably test the watch at an Apple Store before purchasing.

This article originally appeared on Time.com.