A doctor who helped a passenger on a Delta flight Tuesday said she believes flight attendants who repeatedly questioned her credentials even after she showed them her medical license did so because she is a black woman.

Dr. Fatima Cody Stanford was on a Delta flight from Indianapolis to Boston when the woman next to her started hyperventilating, according to NBC News. When Stanford started to help the passenger, a flight attendant came up to ask her if she was a medical doctor. She showed the woman her license without being asked because, she told the New York Times, she knows she “doesn’t look the part.” Stanford, a doctor at Massachusetts General Hospital and an instructor at Harvard Medical school, said she carries a wallet-sized version of the license at all times for that reason.

The flight attendant inspected her license and walked away. Then, according to Stanford, a second flight attendant walked up to her and asked to look at her license. Stanford complied, and the second flight attendant walked away. Then the two flight attendants returned, and one of them asked, “Are you actually an M.D.?” The second woman asked twice, “Is this your license?”

https://twitter.com/fstanfordmd/status/1057433517179703296

Stanford would later complain to Delta on social media about the experience, and Delta apologized to her on Wednesday. After talking to the company, she wrote on Twitter, she learned that the flight attendants had thought she was a therapist, despite her license making it explicitly clear that she is a medical doctor. On Thursday, a Delta spokesman told the Times that the flight was operated by Republic Air, one of Delta’s regional partners, and that as of 2016, flight attendants are not required to verify credentials of people claiming to be medical professionals.

That rule resulted from a 2016 incident in which a black doctor offered to treat a sick patient on a Delta flight and was treated with skepticism after flight attendants demanded “credentials.” According to NBC, Stanford had just interviewed that doctor, Tamika Cross, on Oct. 19 at a conference on medical bias.