On Wednesday, marijuana stocks like Tilray, TLRY, and Cronos Group, CRON, shot up, only to fall again on Thursday, triggering renewed discussion on the growing cannabis industry. For all of that talk and despite rapid growth, the prospect of hefty returns and increasing support for recreational cannabis, the industry still faces a big problem in the U.S.: banks.

Even though recreational cannabis is legal in nine states and the District of Columbia, and legal for medical use in more than 20 others, it is still classified as a Schedule I drug by the federal government. That means that banks don’t want anything to do with money from the cannabis industry. Selling cannabis violates federal law and banks that handle money from cannabis transactions could be charged with money laundering or face prosecution for “aiding and abetting” a federal crime. That’s a huge legal risk.