Before confirming his attendance at this weekend’s Netroots Nation conference in Philadelphia, members of Sen. Bernie Sanders (I-VT) team raised concerns about one of the moderators who would be asking questions at a presidential candidate forum on Saturday, according to two sources who described the conversations to The Daily Beast.

The campaign’s questions centered on Markos Moulitsas, the founder and publisher of the progressive activist website Daily Kos, who has been publicly critical of the Independent from Vermont.

“We discussed the moderators after we announced,” Netroots spokesperson Mary Rickles said of conversations with his team. “There was a question about why we chose who we chose. I told them that both Markos and Cheryl have every intention of being unbiased moderators.”

The forum, hosted with Daily Kos, is set to be moderated by Netroots Nation Board of Directors Chair Cheryl Contee and Moulitsas. Presidential candidates who had polled well in the website’s straw polls of the field, including Sanders, were invited to attend as early as April. The conference, held yearly for over a decade, has played host to activists, campaign workers, volunteers and many of the most prominent Democratic presidential candidates. For candidates like Sanders, and his competitors, it presents the opportunity to interact directly with an important liberal and active base of the party.

As of late Wednesday afternoon, the candidates listed as attending were former Housing and Urban Development Secretary Julián Castro, Governor Jay Inslee (D-WA) and Sens. Kirsten Gillibrand (D-NY) and Elizabeth Warren (D-MA). Warren is the only candidate among the top polling five who has committed to attending.

Sanders will not be able to attend due to scheduling difficulties, though his campaign announced that national co-chair Nina Turner would be there on Friday.

“We have a long and positive relationship with Markos, Mary, Eric, and the whole Netroots team,” an aide to the senator said. “We of course trusted Markos’s commitment to neutrality, and we confirmed as much to them. Although Bernie’s schedule precludes his ability to attend, we wish the organizers and attendees a successful weekend.”

Sanders’ team had gone from a solid “no” last Friday, to, just a few days later, trying to figure out if they could make the event work. One source involved in the scheduling said the moderator was not a dealbreaker.

Rickles noted as well that neither of the moderators are coming up with the questions themselves. Instead, there have been over a thousand submitted questions from members of the community which will drive the conversation with the candidates. Additionally, candidates will be asked two questions from local activists that are expected to come onstage and address the contenders directly.

“I reassured them our goal was to put on an interesting and fair forum,” Rickles said. “No one has the intention of making this forum a place where anyone is going to feel that they are not being treated fairly.”

Moulitsas has been critical of Sanders recently, which could have been the impetus for concern. In a recent interview on Meet the Press, he called Sanders “divisive.”

“The problem with Bernie Sanders is that he has the exact same message he had four years ago,” he said.

That prompted a response from both Sanders’ campaign manager Faiz Shakir and a tweet from the senator himself that appeared to be a direct reference to the criticism.

Earlier this year, Moulitsas took issue with Sanders’ decision to appear on a Fox News town hall saying that it showed the senator was “not a team player.” Even before Sanders launched his second bid for the White House, Moulitsas was quoted as saying: “I don’t see a lot of lasting energy for Bernie.”

Those involved with the planning of the weekend conference stressed that there would be no biases demonstrated, and pointed to criticisms Moulitsas made about other Democrats.

Netroots typically operates as a venue for activists to aggressively seek answers from candidates, which has resulted in some contentious exchanges in the past. Sanders was among the candidates who saw this for himself in 2015 when Black Lives Matter activists interrupted him and Martin O’Malley. Since then, the conference organizers have tried to make a more concerted effort to be more inclusive to people of color and devote even more of the programming to issues affecting these communities.

“Our folks are very vocal. I think they’re going to want to hear some good meaty answers to questions,” Rickles said.

Sanders is far from the only top-tier candidate to miss the event. South Bend Mayor Pete Buttigieg will not be able to make it and former Vice President Joe Biden, whose campaign headquarters are in Philadelphia, is not attending either.

Update 8:30 p.m.: This story has been updated to include comment from Sanders' team.