Google LLC will refund advertisers for ads purchased through its ad marketplaces that ran on websites with fraudulent traffic, The Wall Street Journal reported.

The online ad company AdTrader Inc. sued the Alphabet Inc. unit in California federal court for withholding back payments from 2017 when it offered limited refunds to marketers and ad agencies who had purchased ad space through its online ad-buying tool after the company discovered ads were running on sites whose traffic was artificially boosted by bots.

The Journal said that although Google offered to refund the platform fee for its ad-buying tool, Display & Video 360, it did not cover the total cost of the ad purchases. AdTrader said some of the fraudulent traffic went to Google-owned ad marketplaces such as AdX and AdSense.

A Google engineer said in court documents reviewed by the Journal that refund payments of up to $75 million were pending in connection with its own applications.

Google said in a statement quoted by the newspaper that it planned to refund its clients linked to its own marketplaces before the suit was filed.