The smartwatch brand Fitbit has been hit with a class-action suit filed by customers who say the gadgets’ measures are wildly — and dangerously — off.

The San Francisco federal suit claims that the devices’ heart-rate monitors are inaccurate.

“The heart-rate monitoring function of the PurePulse Trackers is a material — indeed, in some cases, vital — feature of the product,” the lawsuit states.

One plaintiff said her Fitbit measured her heart rate at 82 beats per minute while a personal trainer measured it at 160 bpm.

Lawyer Jonathan Selbin said he hopes to get refunds for unhappy customers or at least partial refunds for customers who bought the more expensive watches with heart monitors.

Fitbit fired back and said the civil suit has no merit and the company “plans to vigorously defend the lawsuit.”