Yale University economics professor Robert Shiller warns against dumping stocks because of his high market-valuation metric but also warns that the bull stock market that has been raging since Donald Trump’s election victory may be flashing warnings that it is about to end.

“Long-term investors shouldn’t be alarmed and shouldn’t avoid stocks altogether,” he wrote for the New York Times.

“But my bottom line is that the high pricing of the market — and the public perception that the market is indeed highly priced — are the most important factors for the current market outlook. And those factors are negative,” he wrote.

“We don’t know where the market will go this month or this year. It could well rise a lot. But investors should not let themselves be tempted to bet aggressively on the Trump bull market,” he wrote.

“The stock market has generally been buoyant in the opening weeks of the Trump administration. The bullish mood could be a self-fulfilling prophecy and lead to continuing gains for a whil,” he said.

“Yet important measurements — some of which I developed — tell us that the market is quite expensive and that investor optimism is tinged with plenty of worry. None of this tells us where the market is going tomorrow, but it suggests that some caution is advisable, and that returns over the next decade or so are likely to be constrained,” he said.

He said is own Shiller CAPE ratio is sending a troubling message.

Shiller’s measure, created with Harvard University economist John Campbell in the 1990s, is called the “cyclically adjusted price-earnings,” or CAPE ratio. The index, sometimes called the “Shiller P/E,” essentially divides share prices by the average of 10 years' earnings adjusted for inflation. It compensates for extreme volatility by valuing share prices based on 10 years of earnings, rather than one year.

“The ratio is now almost 30. Using monthly data, it has been higher only in 1929, when it reached 33, and in the few years around 2000, when it reached 44. In both instances, sharp market declines followed those very high readings. The current level of CAPE suggests a dim outlook for the American stock market over the next 10 years or so, but it does not tell us for sure nor does it say when to expect a decline. As I said, CAPE is useful, but it does not provide a clear guide to the future,” he said.

To be sure, other respected economic gurus say there is a way to use the Shiller index.

"From one year to the next, earnings can vary widely," says Newsmax Finance Insider John Mauldin, chairman of Dallas-based research firm Mauldin Economics. "The reason you use the Shiller ratio is to smooth out those earnings gaps, and get better historical context," he explained to Reuters.

For those fretting about a lofty Shiller P/E, Mauldin suggests a prudent course of action: Keeping some powder dry, in cash.

"The worst thing investors can do is think that they have to put all their money to work, right away," he says. "No, you don't. If valuations like the Shiller P/E seem high, just wait, and they will come back."

Meanwhile, Newsmax Finance Insider Lance Roberts recently took Shiller's CAPE to task.

"But the debate over the value, and current validity, of the Shiller’s CAPE ratio, is not new. Critics argue that the earnings component of CAPE is just too low, changes to accounting rules have suppressed earnings, and the financial crisis changed everything," Roberts wrote for Newsmax Finance.

Roberts explained that this was a point made by Wade Slome previously:

“If something sounds like BS, looks like BS, and smells like BS, there’s a good chance you’re probably eyeball-deep in BS. In the investment world, I encounter a lot of very intelligent analysis, but at the same time I also continually step into piles of investment BS. One of those piles of BS I repeatedly step into is the CAPE ratio (Cyclically Adjusted Price-to-Earnings) created by Robert Shiller.”