U.S. commercial property prices are on the rise, and some may have gotten so high that landlords may not be able to generate enough rent to justify the costs, Reuters reported.

“I think we’ve bottomed, and we’re going in an upward trend,” said Michael Fascitelli, CEO of Vornado Realty Trust. “The question is what’s the slope of that line going upward. The pricing is indicating a much more robust recovery in three to five years than I think we might have.”

Though low interest rates have helped owners keep their borrowing costs down, low rates also tend to push prices higher. Commercial real estate prices are up 33 percent since the mid-1990s, but still down 18 percent from their 2007 peak, according to the Green Street Advisors Commercial Property Price Index.

And because interest rates are unlikely to fall further, owners are expected to be more dependent on the income their properties generate from rent.

“I worry about rates spiking up without inflation in assets,” said Fascitelli, speaking at a real estate event hosted by De La Salle Academy in Manhattan. “You have to make the money by income going up. The risk is that you do OK on the front rentals for a while, but interest rate rises take all the juice away.”