Billionaire investor Warren Buffett expressed concerns over low and negative interest rates at Berkshire Hathaway's (BRK-A, BRK-B) annual shareholder conference on Saturday.

"It’s not just a problem for insurance companies, it’s a problem for retirees, it’s a problem for anyone that’s stuck with fixed investments and finds that their income is a pittance...and that was something that wasn’t in their calculation fifty years ago" Buffett said.

Policy measures by central banks to stimulate financial markets brought interest rates in the U.S., Japan and Europe to historic lows. Negative rates mean that the borrower gets paid to take on debt, while the issuer pays to loan their cash.

“We’ve got $60 billion [of short-term government bonds], let's say at 0.25%. The difference between 0.25% and -0.25% is not all that great. It’s almost as painful to have $60 million out at 0.25% as it is to have it out at a negative rate," Buffett said.

The European Central Bank first pushed rates into negative territory in 2014 as part of an effort to stimulate debt-ridden European economies. Denmark, Sweden and Switzerland followed suit, dropping their overnight rates below zero. Earlier this year, the Bank of Japan surprised investors by announcing a similar measure, in which certain deposits at the bank would return a negative yield.

As of February, $7 trillion worth of government bonds offered negative rates, more than a quarter of all sovereign bonds issued, according to Bloomberg.

"Anything that reduces the value of having money is going to affect Berkshire, because we’re always going to have a lot of money," Buffett quipped.

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