Bernie Sanders is peeved that several of his presidential campaign's field staffers took complaints about their wages to the press rather than resolve the matter internally.

"It does bother me that people are going outside of the process and going to the media," the Vermont senator told the Des Moines Register on Friday. "That is really not acceptable. It is really not what labor negotiations are about, and it's improper."

Some of Sanders' campaign staffers have raised concerns that their flat $36,000 annual salary works out to less than $15 per hour because the staffers often work long hours, according to internal messages obtained by the Washington Post. While the salary would come out to about $17 per hour for a 40-hour work week, the field staffers said they work at least 60 hours per week, dropping the hourly equivalent to $13.

“I am struggling financially to do my job, and in my state, we’ve already had 4 people quit in the past 4 weeks because of financial struggles,” one staffer reportedly told campaign manager Faiz Shakir in a message.

"We are disappointed that some individuals have decided to damage the integrity of these efforts. We are involved in negotiations. And some are individuals that have decided to damage the integrity of that process before they were concluded," Sanders said.

Sanders' campaign became the first ever to approve a union contract for presidential campaign workers in May, with staff represented by United Food and Commercial Workers International Union Local 400.

Sanders added that the union contract "provides, I think, the best healthcare benefits that any employer can provide for our field organizers," and staffers also get paid vacation and sick leave.