Prominent stock-market bull Jeremy Siegel says the best bet in a severe market downturn may be owning 30-year, U.S. government paper.

“And don’t forget, the long bond is the ultimate hedge against a stock-market crash. If there’s going to be a bad event internationally, North Korea, Europe or whatever, everyone runs to the long bond,” the University of Pennsylvania economist and professor of finance at the Wharton School of Business told CNBC during a Monday interview.

“It’s an insurance policy, it gives [the bond] a premium and a premium means a low yield on that instrument,” he said. “If something happens, they will go up if the stock market goes down 500 points.”

Siegel, who is known for his ultrabullish calls on U.S. equity markets—he predicted that the Dow Jones Industrial Average would hit 20,000 at the end of 2015—isn’t offering a forecast for a market crash. In fact, he thinks “equities are a place to be for the investor,” and sees a low risk for a ramp up in tensions between the U.S. and North Korea, who he described as “saber-rattling.”

But he does believe that the 30-year long bond TMUBMUSD30Y, 1.438% , which bears a yield of about 3.01%, compared with a yield of 2.01% for S&P 500 index SPX, -0.46% stocks and a yield of 2.33% for the 10-year benchmark Treasury note TMUBMUSD10Y, 0.678% , would be a compelling investment for investors. Bond prices move inversely to their yields.

Part of Siegel’s thinking is that a surge in anxieties on Wall Street would send the price of government paper higher, pushing yields lower. But until fear jumps, investors will be compensated for holding long-term, 30-yer bonds.

Siegel’s comments on Monday came as lofty equity valuations and a recent spate of lackluster economic reports have raised questions about the sustainability of U.S. equity benchmarks’ rally, which have gained on the back of promises of tax cuts and other pro-growth stimulus pledges from President Donald Trump. Bond yields, however, have been hanging around relatively low levels, despite plans by the Federal Reserve to normalize interest-rate policy off ultralow levels—a plan that should nudge yields higher.

The biggest winners and losers under Trump's tax plan

On Tuesday, tech-heavy Nasdaq Composite Index COMP, -1.25% , which touched the psychologically significant level of 6,000 just last week, hit a fresh closing record of 6,100.72, suggesting that appetite for assets perceived as risky remains healthy. Technology stocks are viewed as a proxy for risk appetite.

Meanwhile, the S&P 500 index SPX, -0.46% was trading steady, not far from its previous closing high set on March 1, while the Dow Jones Industrial Average DJIA, +0.13% bounced around as Apple Inc.’s AAPL, -2.95% hit an all-time trading high and was set to report its quarterly results after Tuesday’s close.

Check out a portion of Siegel’s CNBC interview below: