While Twitter CEO Elon Musk said he purchased Twitter on the basis of protecting people’s freedom of speech, news broke Friday that the microblogging platform is now censoring links to online newsletter platform Substack — showing users an error message when they try to like, reply, comment or retweet.

“Some actions on this Tweet have been disabled,” the error message reads, spurring many to say Twitter is starting a war with Substack.

“We’re disappointed that Twitter has chosen to restrict writers’ ability to share their work,” Substack founders Chris Best, Hamish McKenzie, and Jairaj Sethi told Mashable. “Writers deserve the freedom to share links to Substack or anywhere else…their livelihoods should not be tied to platforms where they don’t own their relationship with their audience, and where the rules can change on a whim.”

While Substack does not view itself as a direct competitor to Musk’s Twitter, Substack launched a “Notes” feature on Wednesday that some think resembles the microblogging platform.

Twitter’s abrupt blockage of Substack links on its app is reminiscent of its recent choice to suppress content from its open-sourced rival Mastodon. In December its account on Twitter was suspended temporarily and links were banned.

Substack recently tweeted that it is investigating these issues and is “actively trying to resolve this and will share updates as additional information becomes available.”

The company’s founders followed up with another tweet fired at Musk: “Any platform that benefits from writers’ and creators’ work but doesn’t give them control over their relationships will inevitably wonder how to respond to the platforms that do.”

As Twitter’s reinvented Blue subscription service, championed by Musk, continues to flounder with fewer than 300,000 global users, Substack boasts 2 million paid subscribers on its platform. Perhaps Musk is jealous, now worried Notes will infringe on Twitter’s unpredictable future.

According to TechCrunch, there is still a way for Subtack writers to alert and connect readers to their work on Twitter: “If you link to a Substack via a redirected URL, it seems to post without restrictions.”

On Tuesday, Twitter labeled National Public Radio’s main account with a “state-affiliated media” tag, which NPR said was “unacceptable,” unfair, and inaccurate.