Want to watch Taylor Swift’s new music video? First, you’re supposed to sit through a video ad. How much advertisers pay for that ad depends on how many times it’s viewed.

However, almost a quarter of the impressions registered for online video ads are fraudulent, according to a new report from the Association of National Advertisers, which found hackers are faking views with networks of computers called “botnets” that make it seem like an ad’s been viewed by a person, when it was really just a computer.

That means advertisers are losing money on these fake views, says Bill Duggan of the Association of National Advertisers.

“While fraud hurts all of the players, publishers, advertisers, and agencies, it hurts the advertisers the most,” he says.

These phony views come up in nearly every conversation that Lauren Fisher, an analyst with eMarketer, has with brands and agencies.

“It’s even going so far as deterring some people from investing in buying video ads because they are so concerned about the level of fraud that they just don’t want to take the risk of losing money in that manner,” she says.

 Advertisers may lose $6.3 billion to this type of fraud next year, according to an advertisers association estimate.