Nobel Prize-winning economist Robert Shiller — who called both the technology bubble and the housing bubble — says the bond market is not currently fitting the traditional definition of a bubble. But he says that long-term bonds are a risky choice right now.

Shiller tells CNBC that he defines a bubble as “a social epidemic of enthusiasm and excitement spread by word of mouth, attracting more and more investors in a market.” He adds, “But I don’t know that the bond market is really driven by excitement. Excitement of sorts — but it isn’t so optimistic. I think it’s mixed with a tinge of regret about ‘Why am I getting such low yields?'”

Shiller says that central banks aren’t the only reason for the ultra low fixed income yields. “The story is longer and deeper than that; it’s not just central banks,” he said. “It’s something about our investment opportunities and our fears and our culture, so it’s a very deep phenomenon. And the question in my mind is: Will it last?”

While he wasn’t willing to offer a firm prediction, Shiller was skeptical that we are in a new normal of low yields. “We can’t go below zero, not far below zero,” he said, referring to the fact that many bonds are trading near 0% on an inflation-adjusted basis. “It seems to me this ‘new normal’ culture could last, but then it could crash.” He adds, however, that he doesn’t think such a turning point is imminent.