The Museum of the City of New York‘s (MCNY) bold historical survey “City as Canvas,” an exhibition drawing on the extensive street art collection of the late Martin Wong and featuring works by Keith Haring, Futura 2000, Lee Quiñones, Rammellzee, and other graffiti legends, has been one of the most popular New York museum shows of the summer. Its run at the Upper East Side institution was even extended, through September 21. But it has one high-profile detractor. In an interview with the Wall Street Journal, the commissioner of the New York Police Department William Bratton took a verbal billy club to the museum for praising an art form that he considers to be mere vandalism.

“I find it outrageous that one of the city’s museums is currently celebrating graffiti and what a great impact it had on the city,” Bratton told the Journal. He was further incensed by the idea of “having New York City school kids at the impressionable age of 12 years old walking through looking at this stuff and having it advertised as ‘Isn’t this great?'”

The director of the MCNY sought to assuage the NYPD commissioner’s fears, telling the Journal: “We are not in the business of trying to encourage children, teenagers, grown-ups or elders to do graffiti.”

Weighing in on the dispute, the curator of another major historical street art show, “Art in the Streets” at MOCA Los Angeles—which included loans from the MCNY—Jeffrey Deitch offered some historical context.

“The young people who invented this style didn’t have access to art supplies,” Deitch told the Journal. “They didn’t have a garage at home where they could work on canvases. They had the streets.”

“City as Canvas” continues at the Museum of the City of New York through September 21.