Black Twitter has a bone to pick with Bernie Sanders.

The 2016 lefty darling’s seeming unwillingness to talk about race on the campaign trail is not sitting well with some black activists and observers, and a tense moment turned #BernieSoBlack into a dominant trending topic over the weekend.

Confronted Saturday by protesters at the Netroots Nation conference in Phoenix chanting “Black lives matter!” and “What side are you on?” Sanders was clearly annoyed. “Black lives, of course, matter,” the populist and self-described socialist senator said. “I spent 50 years of my life fighting for civil rights and for dignity…If you don’t want me to be here, that’s OK. I don’t want to outscream people.”

As a senator from Vermont, which is roughly 95 percent white, Sanders has had scant direct experience connecting with black constituents and voters, and he prefers to focus his speeches on income inequality. His campaign events are overwhelmingly white, and his own advisers have conceded—and he himself has acknowledged—that he is still a virtual unknown among African-American voters.

“Basically, the Bernie Sanders campaign has a lot of groundwork to do with regards to building the relationship with black voters,” Terrell Starr, a senior editor at Alternet, told The Daily Beast. “There is still privilege with how white progressives view progressive politics with regards to racial discrimination…And what we saw [Saturday] was an example of that disconnect.”

Black Twitter went after the senator after his Netroots appearance:

(It’s true: Sanders marched with Martin Luther King Jr. in 1963 and was also arrested for protesting in favor of desegregating public schools in Chicago.)

That @AngryBlackLady tweet, sent out to her 35,000 followers, inspired Roderick Morrow—who runs the “Black Guy Who Tips” comedy podcast with his wife, Karen—to launch the mocking hashtag #BernieSoBlack:

#BernieSoBlack quickly began trending on Twitter:

Morrow, who goes by @rodimusprime on Twitter and has more than 11,000 followers, said he was surprised his impromptu hashtag suddenly took off. “I just thought it was a funny joke!” he told The Daily Beast. But he added that his lighthearted jab was rooted in a serious concern about Sanders’s candidacy.

“Every time race is brought up, he pivots to the economy, which obviously a lot of racial disparity comes via economic means, but some of it is just flat out racism and discrimination,” Morrow said. Sanders’s view that “if we had more jobs in Ferguson, this wouldn’t have happened, I’m not sure that is valid. I mean, Mike Brown was on his way to college. It’s not just a jobs thing.”

The Sanders backlash could indicate friction and dissonance between two critical components of the Democratic base: white liberals and African-Americans. Many younger black activists are skeptical not just of Sanders but of the current Democratic field, who they believe have taken the solidly blue African-American vote for granted.

“Not to disrespect what he did with Martin Luther King,” Morrow said. “But if you want to connect with black voters, they need to know what you are going to do now and going forward, not what you’ve done in the past.”

Still, Morrow said he thinks the backlash this weekend—on Black Twitter, and otherwise—has already done some good.

On Sunday, the official Bernie Sanders Twitter account addressed #BlackLivesMatter directly.

Morrow said he hopes those tweets are a signal the senator will discuss race more frequently during the long campaign. “Well, he’s listening and paying attention,” Morrow said. “The last thing I want for any candidate is for them to give up on the black vote, because that would be the most destructive. Criticism is just part of the job.”