Score a discount on Amazon? You might’ve unwittingly paid more.

More than a quarter of vacuum cleaners sold on Amazon have at some point pretended to offer a discount when they had actually just increased the price, according to new research from Jinhong Xie, JCPenney Eminent Scholar.

By pairing a price increase with the introduction of a previously unadvertised “list price” for a product, Amazon signals to shoppers that they are receiving a discount when they actually pay 23% more, on average, for their new vacuum than they would have just a day earlier. Days after the price hike, the price drops and both the list price and misleading discount claim disappear.

“When you see this list-price comparison, you naturally assume you are getting a discount. It’s not just that you didn’t get a discount. You actually paid a higher price than before the seller displayed the discount claim,” said Xie.