Substack, the popular newsletter platform, recently sent out a push alert for a Nazi-themed newsletter. The publication in question, NatSocToday, describes itself as a “National Socialist weekly newsletter featuring opinions and news important to the National Socialist and White Nationalist Community.” The newsletter’s image header is a Nazi flag, and its latest post, as of Wednesday, was an article that includes the sentence: “We demand the return of all territory currently occupied by jews and non-Whites in historically White homelands.” It does not appear to be a particularly popular blog, and currently has fewer than a thousand subscribers.

User Mag, a newsletter on Substack started by Taylor Lorenz, first reported that a push alert for the publication “was sent to an undisclosed number of users’ phones on Monday.” Some of those people who reportedly received the push alert subsequently posted about it online, expressing confusion over “why they were being prompted to subscribe to a Nazi blog,” the outlet writes.

The story quotes a number of social media users who claim to have gotten the push alert. “I had [a swastika] pop up as a notification, and I’m like, wtf is this? Why am I getting this?” one of them told the outlet. “I was quite alarmed and blocked it.”

Another user commented: “I didn’t realize there was such a prominent presence of the far right on the app.”

User Mag also writes that users who clicked on NatSocToday’s profile were given recommendations for other white-power-themed content.

When reached for comment by Gizmodo, a spokesperson for Substack provided the following statement: “We discovered an error that caused some people to receive push notifications they should never have received. In some cases, these notifications were extremely offensive or disturbing. This was a serious error, and we apologize for the distress it caused. We have taken the relevant system offline, diagnosed the issue, and are making changes to ensure it doesn’t happen again.” The company does not appear to have commented directly on the content of the push alert to NatSocToday. We will update this post when we receive more details from the platform.

In the past, Substack has been criticized for hosting far-right content. However, its willingness to publish a broad variety of viewpoints has also been a selling point. The company, which has mostly subsisted on a subscriber model, initially differentiated itself by largely ignoring content moderation. While it fielded criticism for this, it’s come to be seen as an important venue for a scattered and changing media industry. In April, the site said it would offer legal support for any of its writers, should they be targeted by the Trump administration. It also recently raised $100 million in a funding round, with the New York Times reporting that the site is looking to expand by building out its social network while also potentially populating its site with ads.