Uber is being sued by Fetch Media, the mobile ad agency it launched legal action against last September for failing to prevent ad fraud.

Now the Dentsu-owned agency is demanding Uber cough up $19.7 million for unpaid invoices, arguing that its contract didn’t require it to police or prevent ad fraud from suppliers.

The legal row began in September last year, with Uber alleging Fetch breached its contract by billing the ride hailing company for non-existent ads and taking credit for app downloads it didn’t generate.

At the time Fetch Media strongly denied the allegations and argued Uber hadn’t been paying smaller suppliers Fetch was using to place Uber’s ads.

Reuters reports that Uber voluntarily dismissed that lawsuit on December 22, two weeks after the case was reassigned to U.S. District Judge Yvonne Gonzalez Rogers.

But now Fetch has filed a lawsuit at the same Californian federal court asking Judge Rodgers to get to the bottom of the matter and determine both parties contractual responsibilities.

Who can we blame (and sue) for ad fraud?

The litigation serves as a test case to examine who should take the blame for ad fraud — vendors, agencies or clients?

According to Fetch it wasn’t their job:

“Fetch brings this action to resolve ongoing contractual disputes with Uber, including Uber’s incorrect position that under the parties’ agreements Fetch had a duty to personally police all ad fraud from suppliers that provided mobile advertising services on behalf of Uber,” the company argued in court documents.

Elsewhere in the court documents Fetch says it discussed the ad fraud risks with Uber and recommended the company deploy specific tools to combat ad fraud, and recommended avoiding certain suppliers that presented greater fraud risks. It argues Uber didn’t take their advice.

Uber hired Fetch in 2015 and the two companies had a good relationship for two years, the “Fetch’s advertising successes helped register over 35 million new Uber riders, and Uber gave Fetch the highest possible ratings for its work.”

In 2017 a new team at Uber took over the relationship, which Fetch says “apparently looking to increase their stature within the company, began pointing fingers at Fetch and encouraging Uber to begin handling more of its mobile advertising services in-house.”

Fetch alleges when the relationship ended Uber owed them almost $20 million.