Oaktree Capital founder Howard Marks Bloomberg Howard Marks, billionaire investor and founder of Oaktree Capital Management is adamant about his stance on cryptocurrencies like bitcoin, Ether and others: “They’re not real.”

Marks reiterated this three times to clients Wednesday in his latest Oaktree memo.

“I’d guess these things have arisen from the intersection of (a) doubts about financial security — including the value of national currencies — that grew out of the financial crisis an (b) the comfort felt by millennials regarding all things virtual,” writes Marks. “But they’re not real.”

Marks has clearly done his homework. The 22-page letter is full of research notes and quotes from the New York Times, Wall Street Journal and others about blockchain-based currencies. But he’s still unconvinced that they hold any long-term promise.

“Some people are eager to speculate on digital currency for profit,” he writes. “Others want to put a little money into these to-date-profitable phenomena rather than run the risk of missing out. But they’re not real!”

It all comes down to optimism, says Marks. As soon as our current bull market takes a nosedive, bitcoin and ether speculators could take a big hit.

“They’re likely to keep working as long as optimism is present,” writes Marks. “But their performance in bad times is far from dependable. What will happen to Bitcoin’s price and liquidity in a crisis if people decide they’d rather hold dollars (or gold)?”

Marks isn’t the only old-school Wall Street player to be skeptical of digital currencies. Major banks like Morgan Stanley have recognized bitcoin’s use as a value-holding asset, but are hesitant to call it a true currency.

For people like Marks, Bitcoin has a steep learning curve. It’s not as simple as traditional fiat currency, with notes backed by a central bank and transferred through clearinghouses. Instead, bitcoin utilizes a blockchain, which can instantly credit and debit ledgers of parties involved in a transaction.

It looks like Marks has some more reading to do to catch up.

“Nobody has been able to make sense to me of these currencies,” he writes.