Buffett says he can't predict how negative interest rates will impact economy

In this May 4, 2015 file photo, Berkshire Hathaway Chairman and CEO Warren Buffett speaks during an interview with Liz Claman on the Fox Business Network in Omaha, Neb. (AP/Nati Harnik, File)

OMAHA, Neb. -- Investor Warren Buffett says he can't predict how having interest rates negative for a prolonged period will affect the economy because it has never happened before.

The billionaire from Omaha said on CNBC Monday that Berkshire Hathaway's insurance units in Europe will lose money on the cash they have to keep in banks.

He joked that Berkshire would be better off stashing cash in a giant mattress instead of banks if only he could find a trustworthy person to sleep on top of a billion euros.

Buffett says he understands why regulators cut interest rates to help Europe recover, but there's no way to know how that will affect business.

Buffett released his annual letter to Berkshire shareholders over the weekend.