Bernie Sanders is extremely popular among independents, but he may lose many potential votes in California due to a confusing voter registration issue.

Sanders and his supporters are hopeful for a big win in the delegate-rich state in June, but thousands of independents may have accidentally registered with the far-right American Independent Party — which would leave them unable to vote in the Democratic primary.

“Some independent voters have unwittingly registered for a far-right extremist group that goes by the name of the American Independent Party," Robert Cruickshank, senior campaign manager for Democracy for America, a progressive group supporting Sanders, told AlterNet. “That party was founded to support the racist, segregationist George Wallace during his campaign for president all the way back in 1968. It stays on the ballot only because voters sometimes choose it when they register, thinking they’re registering as an independent."

This could be a major blow to the Senator, as independents strongly favor Sanders. This week’s poll from NBC News|SurveyMonkey found that in head-to-head matchups between Sanders and Donald Trump, the Senator wins by an overwhelming 52% to the former reality television star’s 30%. In the same poll, Trump beats Clinton among independents 39% to her 37%.

The California voter registration offers six parties to choose from, American Independent, Democratic, Green, Libertarian, Peace and Freedom, and Republican — and then options for “no party preference” and “other.” In order to vote in the Democratic Primary, independent voters must be registered as “no party preference.”

Despite the relative obscurity of their platform, as of 2016, approximately 3% of California voters were registered with AIP, making them the 3rd largest party in the state.

AIP is an anti-liberal party — with strong anti-choice, anti-same sex marriage, anti-immigration, pro-gun, and pro-war stances. The party was also once segregationist, but insists that it is no longer one of their platforms.

“We’re not segregationist anymore,” Markham Robinson, AIP executive committee chairman, told the Los Angeles Times. “What we are now is a conservative, constitutionalist party.”

To check if a California voter has mistakenly registered as AIP, and to change to “no party preference,” residents can visit registertovote.ca.gov before the May 23 deadline.