One of Warren Buffett's favorite stocks has just been attacked by HBO comedian John Oliver.

DaVita (DVA) operates dialysis centers, and Buffett's Berkshire Hathaway owns a nearly 20% stake in the company. It was the subject of a scathing 24-minute long segment on Sunday's "Last Week Tonight."

Oliver accuses DaVita of pushing patients to continue receiving long and expensive dialysis treatments instead of having kidney transplants, which could eliminate the need for dialysis.

Four staffers of Oliver's show went undercover to informational "Kidney Smart" classes at DaVita centers.

In several cases, the staffers were urged to get dialysis over a transplant -- even though transplants tend to lead to longer survival rates for those suffering from kidney disease.

Oliver played footage of DaVita CEO Kent Thiry comparing the assembly-line type nature of dialysis centers to Taco Bell and other fast food chains. Oliver said Thiry owed Taco Bell an apology.

Oliver also referred to Thiry as a "showboating musketeer" who "dresses like an idiot" and made fun of his love of the Leonardo DiCaprio film "The Man in the Iron Mask." (It is a comedy show after all.)

But Oliver ended his segment with a plea to viewers to donate their kidneys and other organs.

He even suggested the hashtag #whenidietakemykidneys, which trended on Twitter Sunday night and Monday morning. (HBO is a sister company of CNNMoney. Both are owned by Time Warner.)

But a spokesperson for DaVita said in a statement to CNNMoney that "we are proud of our differentiated clinical outcomes, our teammates' dedication to patient care and our strong culture."

DaVita added that "our teammates are passionate about delivering high-quality patient care and enabling our patients to live fulfilling lives."

"We will continue to advocate for our patients and invest in our teammates and our culture," the company said.

Fresenius (FMS), a German-based owner of dialysis companies that was also mentioned on the show, was not immediately available for comment. Shares of both companies fell Monday though.

Related: Buffett talks about 'huge, huge, huge error' at Wells Fargo

Buffett's Berkshire Hathaway was not available for comment either. But the stock has been a loser for Buffett lately, falling 12% over the past year.

And DaVita, which is the 12th largest holding of Berkshire Hathaway, is the latest Buffett investment to come under fire for controversial practices.

One of Buffett's biggest investments is in Wells Fargo (WFC), the scandal-ridden bank that set up fake accounts.

Berkshire also owns shares of airlines United (UAL), American (AAL) and Delta (DAL), all of which are dealing with PR nightmares after videos surfaced of employees having confrontations with passengers.