Former Treasury Secretary Tim Geithner said he would not invest in bitcoin, even though he doesn’t have a full grasp of how it works.

“No, I wouldn’t,” he said when asked whether he would acquire bitcoin during an interview with Fusion’s Jorge Ramos.

“It depends what your objective is,” he added. “As a way to save money, as an investment vehicle? No. It’s a very volatile thing. As a way to make payments? It’s got some risks because its value fluctuates hugely over time. So, it’s got a lot of challenges.”

Bitcoin has been around since 2009. Over the last year, the online currency has drawn tons of media attention. There’s even a street near Cleveland where you can pay for everything with bitcoin.

Geithner’s stance isn’t necessarily surprising. He once led the agency that prints all U.S. currency and bitcoin is typically used by people who are skeptical of financial institutions and central banks.

But Geithner’s warnings about the cryptocurrency come even though he can’t really explain how it functions.

“I’m the wrong person to ask,” Geithner said. “I don’t think I really understand it.”