Nobel laureate Robert Shiller said he “wouldn’t be at all surprised” if U.S. house prices soon started to decline.

The S&P CoreLogic Case-Shiller index of property values in 20 cities decelerated for a 15th month in June, increasing 2.1% from a year earlier. Shiller noted that adjusting for inflation, prices had already flattened.

“It would suggest declining home prices in the near future,” Shiller, who teaches at Yale University, told Bloomberg Television on Thursday. “I wouldn’t be at all surprised if house prices started falling.”