Former Vice President Joe Biden criticized billionaire Tom Steyer at Tuesday’s debate in South Carolina for investing in private prisons “after he knew that in fact what happened was they hog-tied young men,” in 2004.

“My good friend on the end of this platform, he, in fact, bought a system that was a private prison system after he knew that in fact what happened was they hog-tied young men in prison here in this state,” Biden said, later adding that Steyer was proud of his “accomplishments.”

Steyer immediately took defense and replied, “I bought stock in a prison company thinking they do a better job, and I investigated that, and I sold it.”

Steyer has acknowledged that he sees that investment as “a mistake,” but added that he did not know of the mistreatment of inmates in certain private prisons. He also added that he’s worked to end cash bail in California, telling Biden, “I have worked tirelessly on this, and you know I’m right.”

Steyer sold his stock in the Corrections Corporation of America in 2006, after it was reportedly valued at $90 million. According to Politico, once receiving criticism from Yale University students about the controversial investments, Steyer said in a letter, “We are proud of the work we have done, and continue to do,” for portfolio universities.