House Democrats gave presidential hopeful Sen. Bernie Sanders a hard time in a closed-door session Wednesday after he avoided questions about when he would formally back Hillary Clinton for president, with a group of members booing him at one point, according to several Democratic House members who attended the meeting.

The majority of House Democrats endorsed Clinton’s ticket during the primaries, but Sanders was invited to attend the weekly meeting as a courtesy extended to all of the presidential candidates. When Hillary Clinton was there last month, she was applauded and commended for her commitment to help Democrats regain control of Congress this fall.

Many Democrats have not desired to publicly criticize Sanders for continuing his campaign because they want to make sure that the supporters he garnered will come out and vote for the Democratic ticket in November, but some members let out their frustration with the Vermont senator Wednesday morning, with one member calling Sanders’ appearance before the caucus a “total display of self-obsession.”