Bullishness on Wall Street has soared over the past two months as the stock market has climbed to all-time highs.

Some might even call the prevailing mood “irrational exuberance.”

Regardless of what you call it, it bodes ill for the stock market’s near-term future, according to contrarian analysis. It means that the “Wall of Worry” that bull markets like to climb has largely — if not completely — disappeared. In short, the theory is that if everyone is bullish, there is no one left to buy, and the sellers can take control.

Consider the average recommended exposure level among a subset of short-term market timers who focus on the Nasdaq stock market (as measured by the Hulbert Nasdaq Newsletter Sentiment Index, or HNNSI). Since that market responds especially quickly to changes in investor mood, and because those timers are themselves quick to shift their recommended exposure levels, the HNNSI is my most sensitive barometer of investor sentiment.

This average currently stands at 80.6%, having recently gotten as high as 83.3%. That reading is higher than the highest level to which the HNNSI rose at the tops of the last four bull markets. The average of those sentiment peaks was 60%, far below today’s levels.

An equally telling contrast: The HNNSI fell to minus-55.56% in the panic selling after the U.K. Brexit referendum, indicating that the average Nasdaq-focused market timer was allocating more than half his equity portfolios to going short — an aggressive bet that the market would continue falling.

Contrarians, on the contrary, forecasted a powerful rally, which of course is precisely what ensued.

But with the HNNSI nearly 140 percentage points higher today than then, contrarians have long since stopped singing a bullish tune.

To be sure, extreme levels of bullish sentiment don’t immediately doom the market. On average since I began tracking the investment newsletter industry in the 1980s, bullish sentiment reached its peak between two and three weeks prior to the market itself. This is an average, however, and in some cases the lead time was greater; the longest since the mid-1980s was nine weeks.

Recent high sentiment readings have persisted since mid-July. The HNNSI has been above 70% since July 12, in fact; far from falling, the stock market has held its own. The Dow Jones Industrial Average today is up 0.7% since then; the Nasdaq is 3.7% higher.

Note that even though the five weeks since mid-July are longer than the average lead time between sentiment peak and market top, the elapsed time is still very much within the historical range of past tops. And that means stock market risk is very much above average right now.

Consider the stock market’s returns on those past occasions when the HNNSI was also high. Since 2009, the Wilshire 5000 index has produced an average one-month return of just 0.6% whenever the HNNSI was above 70%. In contrast, whenever the sentiment index was below zero, the subsequent one-month gain averaged 3.3%. An even bigger contrast emerged at the three-month horizon.

These differences are highly significant, at the 95% confidence level that statisticians typically use to determine that a pattern is genuine.

Is there any way the stock market can wriggle out from underneath the indicator’s discouraging message? Of course. Not all measures of sentiment are showing extreme bullishness right now (though most are showing above-average optimism). And, to be sure, sentiment is not the only thing that makes the world go ‘round.

It’s also worth noting that the HNNSI’s extreme could be worked off by an extended trading range market rather than a severe decline. It would take longer in that scenario for the Wall of Worry to be rebuilt, of course, but after a long-enough period in which the market goes nowhere, the bulls would begin to throw in the towel.

A quicker resolution of today’s extreme bullishness would be for a short but severe market drop that reacquaints traders with fear rather than their current greed. If and when that happens, contrarians will return to the bullish camp.