Fitbits were one of the holiday season's most popular gifts. But the makers of the gadget aimed to help you get in shape and stay healthy are now coming under fire.

The makers of the wearable heart rate monitors are facing a lawsuit that claims two Fitbit models (the $150 Charge HR and $250 Surge) do not accurately track heart rates during exercise.

"I'm a mom. I like to work out. I like to be fit," Kate McLellan, one of three plaintiffs in the California lawsuit told TODAY Friday. "My Fitbit was saying that [my heart rate] was at 114, which is really, really low."

After comparing the Fitbit heart rate monitor to those on the equipment at her gym, McLellan said she knew something was wrong. She spoke to Fitbit's customer service and was denied a refund for the product.

Trending stories,celebrity news and all the best of TODAY. This site is protected by recaptcha

"She made it sound like it was my fault, like I was using it wrong or wearing it wrong," McLellan said. "She said it's not really meant to track your heart rate all of the time."

The lawsuit alleges that not only do the Fitbit monitors not work, but also that the company's ads, telling customers "every beat counts," are misleading.

"We do not believe this case has merit," a spokesperson for Fitbit said in a statement. "Fitbit stands behind out heart-rate technology and strongly disagrees with the statements made in the complaint and plans to vigorously defend the lawsuit."

The company is also stressing that Fitbit trackers are designed to provide meaningful data to help users reach health and fitness goals, and they are not intended to be scientific or medical devices.

McLellan's lawyers are seeking a class-action status for the lawsuit, saying the company is treating customers unfairly. The lawyers note that in order to even use the device, you must log in to Fitbit's website and register the heart rate monitor — after you've already bought it.

"You can't even use it as a watch to tell what time it is unless you register it on the website," Bob Klonoff, one of the lawyers who filed the suit, said. "And that's when you have to agree to all of these terms."

That registration includes an agreement that McLellan's lawyers say prevents customers from taking certain legal action against the company.

"They are told they are bound by the arbitration clause and class-action ban," lawyer Jonathan Selbin said. "Well, that's unfair."

Fitbit says its agreement has already been upheld in a California court, and according to their website, customers have 30 days to opt out of the arbitration agreement.

And while many users are stepping up to support the devices, posting four and five-star reviews on Amazon, others are telling this perfect holiday gift to take a hike. This lawsuit is also just one the the latest headaches for Fitbit. The company's stock dropped 20 percent after their latest product, Fitbit Blaze, left investors underwhelmed.