Former New York City Mayor Michael Bloomberg’s presidential campaign on Tuesday said it ended a relationship with a call center company that used prison labor to make calls for the campaign.

The move came in response to a report by The Intercept that one of the campaign’s vendors had contracted a call center company called ProCom. The Intercept reported two of the company’s call centers operated out of state prisons in Oklahoma.

“We didn’t know about this and we never would have allowed it if we had. We don’t believe in this practice and we’ve now ended our relationship with the subcontractor in question,” said a Bloomberg campaign spokesperson.

The Intercept reported that people incarcerated at a minimum security women’s prison, the Dr. Eddie Warrior Correctional Center, made calls to California on behalf of the campaign. The callers didn’t disclose that they were prisoners, according to The Intercept.

The Bloomberg spokesperson said the campaign learned of the issue from the Intercept’s reporting and added that the campaign is asking its vendors to “more properly vet their subcontractors.”

CNN’s Jeff Zeleny reported earlier that Bloomberg has used his considerable financial resources to build out a national campaign for president. He is hiring hundreds of staffers, including top political strategists, and opening offices in key states. He has spent more money on advertising than all of his leading rivals combined.