Former U.S. Federal Reserve Chairman Alan Greenspan said that there is no need for central banks to issue digital currency.

“There’s no point for them to do it,” Greenspan said at Chinese finance magazine Caijing’s annual economic outlook conference.

He pointed out that national currencies are backed up by sovereign credit, something no other organization can offer.

“The fundamental sovereign credit of the United States is far in excess of anything Facebook can imagine,” Greenspan said.

The social network company made waves earlier this year by announcing plans for a cryptocurrency project called Libra that had key partnerships with major global payment processing companies. However, last month Visa, Mastercard, PayPal, Stripe and eBay said they were dropping out of the coalition behind Libra, amid increased scrutiny from the U.S. government.

Meanwhile, the People’s Bank of China has been pressing ahead with work on its own digital currency, although it’s unclear how soon it will be released.