In an opinion piece for The New York Times, Nobel Prize-winning economist Paul Krugman warns about the volatility of the cryptocurrency market and says there are disturbing parallels between crypto and the subprime mortgage crisis 15 years ago, details an article in CNBC.

The subprime crisis resulted when banks gave out high-risk loans with rock-bottom interest rates and skyrocketing housing prices. That saturated the market, and homeowners found themselves upside down in negative equity, unable to repay their loans and garnering huge losses for lenders.

Crypto investors, Krugman writes, are also buying into speculative financial products without a real understanding of the risks. He’s previously compared cryptocurrency to a Ponzi scheme, CNBC reports, and says that crypto is “about as risky as an asset class can get,” because of their enormous price fluctuations. However, given that the entire crypto market is only worth about $1.7 trillion, that’s not a big enough number to pose a risk to the entire system. And indeed, the article continues, Bitcoin and other cryptocurrencies have slipped recently; after a record high in November of $69,000, it’s now around $37,000, a drop of about 46%.