For the past two years, adult coloring books have been big business. Walk into a bookstore, and you’d likely see a rack of them featured prominently by the door. That craze might be coming to an end, however. Barnes & Noble reported its third quarter profits today, which slipped from expectations. The reason? Coloring book and art supply sales have begun to slip.

That’s potentially bad news for physical bookstores. Beginning in late 2015, sales of gimmicky books full of complicated illustrations skyrocketed. Publishers talked about the appeal of the “therapeutic value of coloring,” and worked up ways to sell packaged bundles of products, such as CDs and colored pencils. The result was good for bookstores and the publishing industry as a whole, which saw sales of nonfiction books rise 12 percent during the first half of 2016 compared to 2015. As publishers and brick-and-mortar bookstores face increasing competition from online retailers such as Amazon, the coloring book boost was a welcome change. According to the Association of American Publishers, sales rose by $90 million in the adult books categories.

While ebook sales have largely stabilized in recent years, book sales fell in 2016, which put a lot of pressure on enormous best-sellers such as Paula Hawkins’ The Girl on the Train, J.K. Rowling’s Harry Potter and the Cursed Child, and others to help boost sales. Adult coloring books helped fill the high-sales niche, giving the industry some much-needed good news.

Now the sales increases in coloring books seems to be evaporating. Barnes & Noble noted that the “trends softened” at the end of the year, and the company expects sales to drop in the coming year. To be clear, the publicist I spoke with off the record noted that the books remain a strong seller, but that the market isn’t expanding like it used to, and that publishers are beginning to ramp down their acquisitions as the fad comes to an end.

With the demise of Borders in 2011, Barnes & Noble became the largest book retailer in the United States, and the chain has increasingly worked to increase its presence with online shoppers with its own ebook readers. It’s also started serving alcohol to customers in an attempt to lure them into stores. Don’t count bookstores out just yet. While commentators have been singing Barnes & Noble’s swan song for years now, the chain has held on, and independent bookstores have been doing really well in the digital world. But while it’s easy to mock the novelty of a Harry Potter coloring book, their decline will be felt.