The $9.2 billion he spent buying shares last year made it his largest stock purchase for the company among all of his investment options. He continued buying the stock in the first quarter, adding $2.6 billion in purchases. But last quarter, Buffett spent just $356 million buying the stock, and completely avoided it in June.

The sudden lack of appetite should concern Berkshire Hathaway investors because the stock Buffett's consistently bought until last quarter was Berkshire Hathaway stock itself.

Buffett has been buying shares of Berkshire Hathaway since the 1960s. At first he did so as the portfolio manager for Buffett Partnership Ltd., when Berkshire Hathaway was a struggling textile company. He bought a controlling stake in 1965 and promptly took over as CEO. It took another 46 years before he bought shares as CEO, when Berkshire Hathaway instituted its first share repurchase program, in 2011.

After a long drought in share buybacks due to restrictive language in the repurchase authorization, the Berkshire board of directors changed things in mid-2018. The new repurchase authorization allows Buffett to buy back shares of Berkshire Hathaway whenever he determines the stock trades below its intrinsic value, judged on a conservative basis. The only other caveat is the company must maintain $30 billion in cash or Treasury bills.

Since the change in the authorization, Buffett has bought back shares every quarter. But last quarter's $356 million in buybacks is the lowest amount he's spent so far.

And it's not for lack of cash. Berkshire ended the quarter with a record $277 billion in cash and Treasury bills. Net operating cash flow for the first half of the year came to $24.2 billion (although management warns that number will decrease now that it owes a massive tax bill on all of its stock sales).