Bitcoin is a hot topic, and for good reason: The digital currency, whose value has been on a roller coaster ride this year, crossed the threshold of $10,000 a coin for the first time and then kept going. Some predict it could "easily" reach $40,000 by the end of 2018.

But legendary investor and index fund revolutionary Jack Bogle isn't impressed. He weighed in on the subject at a Council on Foreign Relations event in New York on Tuesday, Bloomberg reports.

"Avoid bitcoin like the plague. Did I make myself clear?" said the Vanguard founder in response to an audience question.

"Bitcoin has no underlying rate of return," Bogle continued. "You know bonds have an interest coupon, stocks have earnings and dividends, gold has nothing. There is nothing to support bitcoin except the hope that you will sell it to someone for more than you paid for it."