While the markets closed today with a slight loss, they opened at a high point. This morning, the Dow broke 26,000 points for the first time in its 120 year history — and that’s after a year with practically no dips to speak of.

But that lack of volatility has some market watchers a little nervous. According to Michael Regan, senior editor of markets at Bloomberg, you never know how long a bull market is going to last. 

“I think the biggest issue that people will worry about going forward is the path of interest rates,” said Regan in an interview with Marketplace host Kai Ryssdal. “It’s very unusual to get this type of economic stimulus in the form of tax cuts at this point in the economic cycle when everything is going pretty well. So there’s a concern that things could overheat a little bit and that inflation will finally really pick up. And that tends to cause interest rates to go higher.”

Click on the audio player above to hear the full story.