Tesla Inc Chief Executive Elon Musk on Thursday called for Amazon.com Inc to be broken up, after an author complained on social media about being unable to self-publish a book via the world’s largest online retailer.

“This is insane jeffbezos,” Musk tweeted at Amazon’s chief. Both men are billionaires and oversee competing space exploration companies on top of their duties at Tesla and Amazon. “Time to break up Amazon. Monopolies are wrong!” Musk added in a second tweet.

The retailer had no comment on Musk’s call for its dissolution but said the book in question, by author Alex Berenson, had been blocked in error and was now available for sale via its Kindle e-reading service. The decision to greenlight the book’s sale was not due to Musk, Amazon said.

According to a photo of a message Amazon sent Berenson, posted on Twitter, the retailer had taken action because the 22-page book did not comply with guidelines that Amazon did not specify, adding, “we are referring customers to official sources for health information about the virus.” Berenson’s book is titled, “Unreported Truths about COVID-19 and Lockdowns: Part 1: Introduction and Death Counts and Estimates.”

Berenson did not respond to an email requesting comment. After Amazon approved the book’s sale, he tweeted, “thanks to elonmusk and everyone who helped.”