Streaming music service Spotify is starting to crack down on users who are running modified versions of the streaming platform to gain the benefits of premium membership without paying, according to Torrent Freak, a website that reports on copyright and piracy news.

Spotify users who have been making use of the illegal versions of the company’s software have started to receive emails from the company claiming there is “abnormal activity” coming from their app. Affected users have had their accounts disabled but not deleted.

“We detected abnormal activity on the app you are using so we have disabled it. Don’t worry – your Spotify account is safe,” the Spotify email reads.

The message advises users to “uninstall any unauthorized or modified version of Spotify and download and install the Spotify app” from an official, verified marketplace like the Google Play Store or Apple App Store.

The crackdown from Spotify appears to specifically target users who are using a modified version of the Spotify app that has been made available online and provides users with the features of the service’s Premium membership, which normally costs $10 per month.

Using the hacked version of the Spotify app, a user with a free account would be able to listen to music without advertisements as well as skip as many songs as they would like. A standard free account is subjected to ads after every couple songs and can only skip six songs per hour.

The hacked versions of Spotify are available in all different corners of the web for users who know where to look. Given the recent scrutiny applied by Spotify, it appears many of those options are getting pushed back into the shadows.

Android Police recently reported one of the most popular cracked Spotify applications—an app called Spotify Dogfood—was hit with a Digital Millennium Copyright Act (DMCA) claim that forced the developers to take the service offline.

Earlier this year, International Business Times reported on the discovery of a cracking tool found online that allowed hackers to hijack existing Spotify accounts. Those hijacked accounts were then often sold online via the dark web to users who pay a fraction of the monthly fee for Spotify’s premium service.

It’s difficult to gauge the scope of Spotify pirating. The company recently reported that it has a total of 159 million monthly active users—71 million of which pay for the company’s premium service. It is likely a very small portion of those 159 million users have taken advantage of the workaround.

No matter how small the percentage, Spotify likely intends to clean up the issue before it officially goes public. Last week, Spotify finally filed for its long-rumored initial public offering (IPO) that valued the company at $1 billion. The streaming platform has thus far struggled to make a profit despite reporting $4.09 billion in revenue in 2017.