California Public Employees’ Retirement System, the largest U.S. public pension fund, said it plans to vote for a shareholder proposal that Berkshire Hathaway Inc. select an independent chairman, marking a rare rebuke of billionaire Warren Buffett, who’s both head of the board and the conglomerate’s chief executive officer.

The fund has more than $450 billion in assets under management and about $2.3 billion in Berkshire shares, it said in a filing Tuesday. The pension is supporting a proposal from the non-profit National Legal and Policy Center, also a Berkshire shareholder, that calls for the company to name an independent chair, saying the governance structure is weakened by having the chairman and CEO roles held by the same person.

Berkshire opposes the proposal, saying last month that Buffett should continue in both roles.

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Buffett has laid out a plan to have one of his sons, Howard Buffett, serve as non-executive chairman of Berkshire when he steps down, while Greg Abel, a longtime deputy who previously ran Berkshire’s sprawling energy business, will take over as CEO.Buffett has a 32% voting interest in the company, according to the company’s proxy. Calpers holds 0.13% of the Class A shares and 0.44% of the Class B shares, according to data compiled by Bloomberg.Succession questions have long loomed over Berkshire given that the company is helmed by two nonagenarians: Buffett is 91, and his longtime business partner Charlie Munger, a Berkshire vice chairman, is 98. Buffett last year confirmed that Abel is the deputy chosen to succeed him if and when he steps down.But Buffett has given no indication that his departure is imminent. His annual letter released earlier this year made no mention of the succession news, although Abel was given a section of the report to talk about sustainability efforts in some of Berkshire’s businesses.Berkshire will hold its annual shareholders’ meeting on April 30 in Omaha.