Sen. Bernie Sanders (I-VT) chastised fellow presidential candidate Donald Trump on Friday for allowing a man to incorrectly claim that President Obama is a Muslim. Sanders took to Twitter to demand the GOP real estate mogul apologize, and said: “This nonsense has got to end.”

Trump must apologize to the president and American people for continuing the lie that the president is not an American and not a Christian. — Bernie Sanders (@BernieSanders) September 18, 2015

Sanders’ comments were a response to an incident on Thursday at a town hall event in New Hampshire, where a man claimed that Obama was a Muslim who was not born in America. The voter then asked Trump about the “problem” of Muslims in America.

“We have training camps growing where they want to kill us,” the man said. “When can we get rid of them?”

Instead of correcting his supporter’s false claims about the President and the training camps, Trump responded: “A lot of people are saying that, and a lot of people are saying that bad things are happening out there. We’re going to be looking at that and plenty of other things.”

Sanders called the exchange “nonsense,” and sent a tweet encouraging an end to xenophobia.

This nonsense has got to end. — Bernie Sanders (@BernieSanders) September 18, 2015

Let’s stop the racism. Let’s stop the xenophobia. — Bernie Sanders (@BernieSanders) September 18, 2015

Other Democrats, including Democratic National Committee Chair Rep. Debbie Wasserman Schultz (D-FL) and Democratic presidential candidate Hillary Clinton, have also condemned Trump’s response.