Apple’s latest mobile operating system, iOS 9, shipped with a pretty cool new feature. It’s called Wi-Fi assist, and it automatically switches you over to cellular data when Wi-Fi connectivity is weak.

Apple products hold on to whatever Wi-Fi you’re connected to pretty well. The signal might be terrible, but iOS will cling to any available Wi-Fi – even if it’s so slow as to render your phone a brick. Instead of simply switching over to cellular, you’ll sit there with your Wi-Fi spinning, trying to load up Google Maps. Or Spotify. or whatever.

Wi-Fi assist solves this problem. Neat.

Except when it’s not.

If you’re watching your data consumption, it could be a problem. Apple’s Wi-Fi assist could be a real data suck. In fact, in the days after iOS 9’s wide release, there was a lot of bitching on social media about battery life.

Wi-Fi assist in turned on by default when you update to iOS 9.

Now, Wi-Fi assist is the focus of a class action lawsuit.

A new lawsuit alleges over $5 million in damages for data costs. The suit, filed in a District Court in San Jose, California, says Apple didn’t properly explain the Wi-Fi assist feature until people started complaining.

At that point, Apple did put up an explainer.

Still, that’s not good enough, according to the lawsuit.

“Defendant’s above corrective action, however, still downplays the possible data overcharges a user could incur.Reasonable and average consumers use their iPhones for streaming of music, videos, and running various applications — all of which can use significant data. Defendant’s corrective statement does not disclose any basis for its conclusion that an average consumer would not see much increase in cellular usage,” says the suit.

If you’re worried about Wi-fi assist and your data it’s easy to disable it. Just head to Settings > Cellular and scroll all the way to the bottom. There you’ll find the Wi-Fi Assist option.