DAKAR, Senegal (AP) — Nobel Prize-winning author Wole Soyinka said on Tuesday that his non-resident visa to enter the United States had been rejected, adding that he believes it may be because he recently criticized U.S. President Donald Trump.

The Nigerian author, 91, won the Nobel Prize in Literature in 1986, becoming the first African to do so.

Speaking to the press on Tuesday, Soyinka said he believed it had little to do with him and was instead a product of the United States' immigration policies. He said he was told to reapply if he wished to enter again.

“It’s not about me, I’m not really interested in going back to the United States,” he said. “But a principle is involved. Human beings deserve to be treated decently wherever they are.”

Soyinka, who has taught in the U.S. and previously held a green card, joked on Tuesday that his green card “had an accident” eight years ago and “fell between a pair of scissors.” In 2017, he destroyed his green card in protest of President Trump's first inauguration.

The letter he received informing him of his visa revocation cites “additional information became available after the visa was issued,” as the reason for its revocation, but does not describe what that information was.

Soyinka believes it may be because he recently referred to Trump as a “white version of Idi Amin,” a reference to the dictator who ruled Uganda from 1971 until 1979.

The U.S. Consulate in Nigeria's commercial hub, Lagos, directed all questions to the State Department press office in Washington, D.C., which did not respond to immediate requests for comment.

Soyinka jokingly referred to it as a “love letter” and said that while he did not blame the officials, he would not be applying for another visa.

“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.”