The epic rally in shares of tech behemoths is much outpacing their profits, and it could mean the S&P 500 is looking more vulnerable, based on Apollo Global Management chief economist Torsten Sløk.

In a note on Sunday, he identified that the highest 10 firms within the S&P 500 account for 35% of the index’s market value but only 23% of its earnings.

“This divergence has never been larger, suggesting that the market is record bullish on future earnings for the highest 10 firms within the index,” Sløk wrote. “In other words, the issue for the S&P 500 today is just not only the high concentration but in addition the record-high bullishness on future earnings from a small group of firms.”

Since the S&P 500 is weighted by market cap, soaring share prices of Big Tech firms rushing into the AI boom has meant that recent gains are concentrated into only a handful of stocks, obscuring the relative mediocrity for remainder of the index.

Before Nvidia began its selloff earlier this month, the AI chip leader accounted for greater than a 3rd of the S&P 500’s rally this 12 months.

“Such a high concentration implies that if Nvidia continues to rise, then things are wonderful,” Sløk warned on June 12. “But when it starts to say no, then the S&P 500 can be hit hard.”

As market leadership becomes more concentrated, so are investors’ portfolios, especially as putting money in funds that track indexes becomes increasingly popular.

Bank of America analysts said in a recent note that the common large-cap fund has 33% of its portfolio in its top five holdings, up from just 26% in December 2022.

Similarly, the share of funds the have greater than 40% of their portfolio of their top five holdings has jumped to 25% from less 5% in December 2022.

Meanwhile, Wall Street analysts have been bullish on the S&P 500 and are scrambling to boost their year-end targets. Even considered one of the largest bears has surrendered and is now one of the crucial bullish analysts.

And Fundstrat Global Advisors cofounder Tom Lee recently said the S&P 500 could hit 15,000 by the top of the last decade. He isn’t the one Wall Street bull making daring predictions.

Ed Yardeni has been pounding the table about one other “Roaring Twenties” super-cycle and has said the S&P 500 would jump to six,000 by next 12 months. By the top of the last decade, he said the stock index could reach 8,000.

This story was originally featured on Fortune.com