Bernie Sanders received just one campaign donation from a billionaire, a $470 check from the wife of automotive technology magnate David Hall.

But it didn’t stay in the Sanders campaign coffers for very long.

As Forbes reported, the campaign returned the check to Marta Thoma Hall, who sent the check for $470 after Sanders called Donald Trump a liar over the summer. Hall said she appreciated the bluntness from Sanders — and wanted to support his campaign — but apparently didn’t know that the Vermont Senator had pledged not to accept any support from Super PACs, or take campaign contributions from billionaires.

Forbes may have actually played a role in the returned check. The business news outlet found out that Hall had donated, and asked for a comment from the Sanders campaign. Sanders’ campaign was apparently unaware that she was married to the man who has pioneered technology for self-driving cars. David Hall just barely made the billionaire cutoff, with Forbes estimating his net worth at exactly $1 billion.

The outlet caught up with Marta Thoma Hall as well, who said that she gives various small political donations to candidates who spark her interest. She claimed to have given $505 to Kamala Harris after hearing the California senator talk about women’s reproductive rights.

Hall told Forbes that she was disappointed Sanders didn’t want to keep her donation.

“I think it’s disappointing,” she said.

“I don’t understand why they would do that. That’s ridiculous.”

The Sanders campaign has been able to manage quite well without any contributions from billionaires. As CNN reported, the Sanders campaign pulled in a massive $25.3 million haul in the third quarter, up more than $7 million from the second quarter. At the time, the campaign bragged that it was not taking money from billionaires.

“Bernie is proud to be the only candidate running to defeat Donald Trump who is 100 percent funded by grassroots donations — both in the primary and in the general,” Sanders campaign manager Faiz Shakir said as the numbers were announced.

Sanders utilized a similar strategy in 2016, building up a base of smaller donors. The same approach was employed by Howard Dean in 2004, and was used to great success by Barack Obama in 2008 and 2012. However, Sanders’ vow not to take any money from billionaires set him apart in 2016, and paved the way for the 2020 campaign. A number of other candidates have now joined Sanders in vowing not to take support from Super PACs.