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It's an indicator which may humble investors riding the record breaking S&P 500 Index's rally.

Jeroen Blokland, a widely followed money manager, finds stocks in the U.S. are too pricey based on a chart comparing the level of the market to the amount of sales generated by the companies within the S&P 500.

The higher the price-to-sales ratio rises, the more investors are paying per every dollar of revenue.

"What you see here is that the price-to-sales ratio has risen to the highest level in 15 years," Robeco Investment Solutions senior portfolio manager Blokland told CNBC's "Futures Now" recently.

"In order to justify these kind of valuations, you must anticipate that for now on that that sales will grow into the valuation," Blokland said. "So, sales growth has to pick up in order to justify this level. So far, we haven't seen that yet."

He acknowledges that growth has been decent in the U.S., but says that it doesn't nearly justify the recent record breaking levels.

So far this month, the has broken its intraday all-time high six times.

A catalyst? Foreign investors have been looking to the U.S. as a growth engine, as geopolitical issues cause uncertainty overseas.

Beyond valuations, Blokland believes the Federal Reserve could pose a problem for the rally.

The growing U.S. economy could push the Fed to raise interest rates this year and surprise a lot of investors, according to Blokland. Right now, the majority believes the Fed won't raise rates until 2017 — after the presidential election.

"If the economy keeps up, why not raise rates? They want to get off that very, very low level and normalize policy a bit," he said. "They have missed out on a couple of opportunities already."