Have you updated the software on your iPhone yet? The latest upgrade is iOS9, and there’s lots to love about it: the usual bug clean-up, a new way to switch between applications, Siri’s improved helpfulness. But as PureWow reports, there’s one new feature that’s hurting more than it helps— and depending on your data plan, it could cost you hundreds.

So what is it? It’s called “Wi-Fi Assist,” and the idea is that when the Wi-Fi signal is weak or your phone is connected to a slow network, the cellular data will jump in to “assist,” attempting to find you a better network and improve your connection. It’s a nice idea in theory, but you see the problem, right? If it’s turned on, it’s running, and that means you could just be sitting anywhere – even at home! – thinking that you’re saving data by connecting to your Wi-Fi, and meanwhile your data is draining away in the background. Scary!

Don’t fret too much, though; turning it off is easy! Go into Settings, then enter the “Cellular” menu. Scroll all the way to the bottom and you’ll see Wi-Fi Assist listed. Toggle it off, and you’ll never have to worry about this costly feature again. What a relief! Do you know anybody who’s been affected by this change?