A court in Amsterdam sentenced a Polish-Canadian national to two months in prison on Thursday for projecting a message alluding to an antisemitic conspiracy theory onto the Anne Frank House museum.

Robert Wilson was charged with insulting a group and inciting discrimination for using a laser projector in February to display the words “Ann (sic) Frank invented the ballpoint pen” on the side of the canal house where the Jewish teenager hid with her family during the Holocaust.

The text refers to a debunked claim that Frank’s famed diary is a forgery.

“Given the great symbolic significance of Anne Frank’s diary for the commemoration of the persecution of the Jews, this statement can be regarded as a form of Holocaust denial,” the court wrote in its decision.

Having already spent more than two months in pre-trial detention, Wilson has already served his sentence. He was not in the courtroom for the verdict.

The judges ruled that Wilson had projected the scrolling text from a van parked across the canal from the building in Amsterdam, which now houses the Anne Frank Museum. A recording of the stunt was posted on an antisemitic Telegram channel, but the court found there wasn’t enough evidence to convict him of distributing the images.

Wilson denied the charges, claiming he was in Amsterdam for a weekend getaway with his girlfriend and daughter. He told judges during a hearing two weeks ago that he wasn’t even aware of where the Anne Frank House was.

Prosecutors said Wilson was a prominent member of the neo-Nazi Goyim Defense League. He is facing charges of assault and shouting homophobic slurs at a neighbor while he was living in the United States. Poland is also investigating Wilson over an incident in which he allegedly stood in front of the Auschwitz concentration camp holding a sign with antisemitic slogans.

Frank kept a diary of life under German occupation in World War II, when, as a Jew, she was in constant danger. She was arrested with her family in 1944 and sent to a Nazi concentration camp, where she died, but the diary survived and became one of the world’s most famous books.

Several pages written with a ballpoint pen were found among Frank’s papers in the 1980s. That type of pen was not introduced in the Netherlands until after World War II, and Holocaust deniers have claimed this proves the diary, published by her father after the war, is fake. However, researchers have concluded that the pages were accidentally left in the diary in the 1960s.