Hot new fitness monitor Fitbit may be in some serious legal trouble.

A class action lawsuit has been filed against the wearable fitness tracker, claiming that Fitbit falsely claims to accurately measure heart rate with its monitoring of “Charge HR” and “Surge,” according to an AL.com report.

Fitbit claims its bands, which range in price between $150 and $250 each, have something called “PurePulse,” which is technology that uses LED to track heart rate during exercise. But the lawsuit claims that they falsely or deceptively use slogans like “Know Your Heart.” Instead, the bands appear to be completely wrong on heart counts, and not by a small margin.

And it’s an issue of safety, not just about consumers getting their money’s worth, the lawsuit claims, pointing to examples of underestimating heart rates resulting in users overexerting themselves dangerously.

Ars Technica obtained a statement from Fitbit that claims the heart rate technology is just fine and the lawsuit is without merit. It did note, however, that the Fitbit devices aren’t supposed to be a scientific or medical device, even if they do provide “meaningful data.”

The full statement: “We do not believe this case has merit. Fitbit stands behind our heart rate technology and strongly disagrees with the statements made in the complaint and plans to vigorously defend the lawsuit. Fitbit is committed to making the best clip and wrist-based activity trackers on the market. Our team has performed and continues to perform internal studies to validate our products’ performance… But it’s also important to note that Fitbit trackers are designed to provide meaningful data to our users to help them reach their health and fitness goals, and are not intended to be scientific or medical devices.”

It’s not the only lawsuit Fitbit is facing. Another suit alleges that Fitbit devices were incorrect in measuring sleep, often underestimating it.