Amazon might be aiming to scale up its cashier-less stores, according to a report in The Wall Street Journal, which says that the company is testing the technology in larger stores.

In January, Amazon opened its first cashier-less store in Seattle, which was followed by additional locations in Seattle, Chicago, and San Francisco. Those stores use cameras and software to detect what items customers pick up and charges them accordingly, allowing them to forgo the entire check-out process.

Amazon’s existing stores are the size of a small convenience store — and are thus much smaller than your typical grocery store, with fewer items and people to keep track of. Amazon’s rollout of the stores were delayed in 2017 because the stores kept breaking when there were more than 20 people inside. The WSJ says that Amazon has improved the software in those stores since they’ve opened, but apparently the technology still has trouble in those “bigger spaces with higher ceilings and more products,” according to the WSJ’s sources.

According to the report, Amazon is testing the technology in “a larger space formatted like a big store.” Those sources also say that the “most likely application” of that technology is for Whole Foods, which Amazon purchased in June 2017. It has said in the past that it doesn’t plan to implement the technology in the chain’s stores.

Amazon reportedly has big plans for its cashier-less stores, with plans to open as many as 3,000 by 2021. That would allow it to compete with chains like CVS and Walmart, and in order to do so, it would need to fix a range of sizes, from the smaller corner store to the larger supermarkets. Other companies are paying attention as well. In October, Walmart announced that it was opening an experimental cashier-less Sam’s Club outlet in Texas, in which customers would use an app on their phone to scan items, rather than relying on the store’s systems to track their purchases.