Nvidia is the most expensive stock in the S&P 500 Index, with its shares trading for roughly 23 times the company’s projected sales over the next 12 months.

But there’s a problem with that valuation.

In the age of the artificial intelligence boom, no one can figure out what the chipmaker’s revenues are actually going to be — not the Wall Street analysts covering Nvidia or Nvidia executives themselves. So how are investors supposed to calculate whether the shares are expensive or not?