The Interactive Advertising Bureau (IAB) has booted Adblock Plus from its yearly conference with little more than “sorry, not sorry.” The company, which boasts more than 400 million downloads of its ad-blocking browser extension, was dismissed with a curt email from the IAB, according to an Adblock Plus blog post .

A screenshot posted to Adblock Plus’s blog shows an email exchange between the company and the IAB, in which a representative of the IAB simply states that Adblock Plus’s registration had been canceled and the fee refunded:

The decision was seemingly out of the blue, given that Adblock Plus had even been included in the participant lineup on the IAB’s website.

“The over 400,000,000 downloads of Adblock Plus are not going to ‘go away,'” the company wrote in its blog post. “Dis-allowing Adblock Plus from attending your event solves nothing. We will proceed to work with others to build a sustainable monetization model for the Internet.”

A tweet from Wednesday morning indicates that the IAB’s tiff is specifically with Adblock Plus; another ad-blocking company, Ghostery, remains invited to the conference. The CEO of Ghostery is set to speak on stage there about the issue of increased ad blocking.

The IAB’s move to shut out Adblock Plus is doubly strange in the context of a related admission by the organization: Soon after iOS started welcoming ad blockers this past fall, the IAB published a post in which it said the advertising industry was largely to blame for the rise of ad blocking.

“We messed up,” the post read. “As technologists, tasked with delivering content and services to users, we lost track of the user experience… The fast, scalable systems of targeting users with ever-heftier advertisements have slowed down the public Internet and drained more than a few batteries.”