Nigerian author Wole Soyinka says that his U.S. visa has been revoked, the Associated Press reports.

Soyinka, the Nobel Prize–winning playwright, poet, and fiction writer, said that he believes the revocation of his visa is because of his criticism of President Donald Trump.

Soyinka, one of Africa’s most celebrated writers, is known for his plays The Lion and the Jewel and The Road and his novels The Interpreters and Chronicles From the Land of the Happiest People on Earth.

Soyinka was at one time a permanent resident of the U.S. but destroyed his green card after Trump was first elected president in 2016. “I had a horror of what is to come with Trump.…I threw away the card, and I have relocated [to Nigeria], and I’m back to where I have always been,” he said. Soyinka recently referred to Trump as a “white version of Idi Amin,” the despot who ruled Uganda in the 1970s.

The Guardian reports that Soyinka received a letter from the U.S. consulate in Lagos informing him that the visa had been revoked. According to the AP, the consulate directed questions about Soyinka’s visa to the Department of State, which did not respond to a request for comment.

Soyinka said, “It’s not about me, I’m not really interested in going back to the United States. But a principle is involved. Human beings deserve to be treated decently wherever they are.…I have no visa. I am banned, obviously, from the United States, and if you want to see me, you know where to find me.”

Michael Schaub is a contributing writer.