Earlier this year, Ed Yardeni said the S&P 500 index could hit 10,000 by 2029.

However, in the wake of the index’s rapid run-up following its tumble this spring, Yardeni is concerned.

He thinks the long trade has gotten too crowded.

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Over the past three years, the bulls have won the day on Wall Street. The S&P 500 index still has a chance to register three consecutive years of at least 20% gains, something that has rarely occurred before. Some strategists and pundits think the party can continue, as the Federal Reserve has cut its benchmark interest rate at each of its last two meetings, and the labor market, though it may be slowing, is seemingly still on solid footing.

But there is another camp of strategists who believe a bubble has formed that could pop with ugly consequences. In the latest indication that conditions have perhaps gotten too frothy, one of Wall Street’s most fervent bulls has started to grow cautious about the state of the stock market.

Many market watchers may have heard of Ed Yardeni of Yardeni Research because he has been one of the most bullish strategists in recent years. In fact, early this year, he said the S&P 500 index could hit 10,000 by 2029, which implies close to another 50% upside over the next four years.

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But in a recent interview with Bloomberg, Yardeni was more reserved and even outright cautious on the state of the stock market, which has risen by close to 15% this year (as of Nov. 6).

“There are too many bulls,” Yardeni said, adding that the market rally may have gotten ahead of itself. “Just one unexpected event could knock stocks down from their highs amid poor market breadth, but that may be tough to do, given that traders are usually optimistic around the holidays.”

One such event could be if the Federal Reserve fails to lower the federal funds rate at its final meeting of the year in December. The Fed cut interest rates by 0.25 percentage points (25 basis points) at each of its last two meetings.

Following the last Federal Open Market Committee meeting, Fed Chair Jerome Powell said that another cut in December was not a “foregone conclusion.” While the experts’ views on the odds of a December cut decreased drastically following Powell’s comments, traders betting on the federal funds rate still see about a 65% chance of a cut. If those odds continue to drop, though, the stock market may slide as well.

Things have been more difficult for the Fed because of the government shutdown, which has resulted in several government agencies not publishing key economic data that the central bankers rely on to guide their fiscal policy moves.