The source of Compass’ complaint, which was filed on Monday in a New York federal court, is a new rule from Zillow stating that any home listing publicly marketed to consumers anywhere must be published on Zillow within one day or it won’t be allowed to be published on Zillow at all.

Compass, the real estate brokerage, claims that Zillow, the website that has 227 million unique visitors a month, has conspired to maintain a monopoly over digital home listings.