Google is issuing refunds to advertisers whose ads ran on its sites later found to have fake traffic, according to a report in the Wall Street Journal.

The refunds are being made while Google works on a tool to give online marketers more transparency into the ads they buy via the company’s technology, known as DoubleClick Bid Manager.

Google CEO Sundar Pichai has said that the company called advertisers to assuage their concerns about ads running next to objectionable content like hate speech or other forms of violence.

The refunds over fake traffic suggest that fake ads are a bigger problem than the company has previously acknowledged.

An email seeking comment from Google, a unit of Alphabet, wasn’t immediately returned.

Read the full report in WSJ.