Bernie Sanders fired up his supporters at the Democratic National Convention on Monday afternoon — until he mentioned the need to elect Hillary Clinton.

While Sanders drew cheers when he trashed Donald Trump during his address, the audience erupted into boos when he said that “we have got to elect Hillary Clinton and Tim Kaine.”

In the run up to his statements about Clinton and Kaine, Sanders touted his campaign’s many achievements, including writing a more progressive Democratic platform and reducing the total number of Superdelegates in Democratic primaries.

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But even after getting the crowd pumped up by talking about their positive achievements, Sanders could not contain the boos that rained down on him when he endorsed Clinton and Kaine. He responded by reiterating how dangerous of a candidate Donald Trump is, and he talked about how Trump would be a disaster for all of the causes he cares about.

“Trump is a bully and a demagogue,” Sanders said. “Trump has made bigotry and hatred the cornerstone of his campaign.”

While this calmed things down a little bit, he didn’t try to make a positive case for Clinton, and with good reason: It’s likely he would have been booed again had he mentioned her name.

Check out a video of Sanders getting booed below.