Bitcoin cannot be considered a currency, one of the world’s most prominent financial commentators and institutional investors said.

Mohamed El-Erian told CNBC that currencies normally serve as a store of value, but that they must be predictable and stable to work as a medium of exchange.

El-Erian, who is currently the chief economic advisor at the German financial institution Allianz, continued:

“Bitcoins aren’t there yet, they’re still trying to find stability so it’s more of a commodity than it’s a currency.”

A former CEO and co-chief investment officer of fixed-income powerhouse PIMCO, El-Erian had previously said in September he believed bitcoin was overvalued, and that its price may reflect an assumption that the cryptocurrency will be adopted widely. At the time, he said that would not happen.

He doubled down on this assertion Thursday, telling CNBC that investors in bitcoin may be waiting for widespread adoption but that those thinking institutions and central banks would begin accepting it could be wrong.

El-Erian said his main long-term concern was whether “the assumption in the pricing about adoption [was] consistent with reality”

“That’s the issue that investors should ask if they’re holding bitcoins for a couple of months,” he told CNBC.

El-Erian is not the first major figure in mainstream finance to view bitcoin as more of a commodity than a currency. The governors of the Bank of Mexico and the Bank of Korea both gave similar assessments this year, as did the U.S. Commodity Futures Trading Commission in 2015.

Mohamed El-Erian image via World Economic Forum / Wikimedia Commons