Mastercard instructed U.S. financial institutions this week to stop allowing purchases of cannabis on its debit cards, stripping customers of a convenient way to purchase cannabis without cash. Cannabis businesses say the decision will increase the risk of robbery and violent crime.

Because federal law prohibits the sale, possession and use of cannabis in all its forms, Mastercard said that purchases were not allowed on its systems, even when customers use bank cards and PINs to access their own cash to buy cannabis in states where the drug is legal for recreational or medical reasons.

“As we were made aware of this matter, we quickly investigated it,” a spokesman for Mastercard, one of the world’s largest payment processors, said in a statement on Friday. “In accordance with our policies, we instructed the financial institutions that offer payments services to cannabis merchants and connects them to Mastercard to terminate the activity.”

Medical cannabis is legal in 38 states, three territories and the District of Columbia. Recreational use of the drug is legal in 23 states, two territories and the District of Columbia.

Because most big banks and credit card providers will not work with cannabis businesses, many operate by cash only, making them targets for a growing number of robberies, particularly when transporting large amounts of cash for deposit.