The recently reopened Royal Museum for Central Africa in Belgium has been caught in a media firestorm for allowing a music festival to take place in which a partygoer reportedly wore blackface while others turned up in clichéd “African” costumes included grass skirts.

Colloquially known as the AfricaMuseum and located not far from Brussels in Tervuren, the colonial-era institution founded byKing Leopold II has worked hard to acknowledge the ruler’s infamous colonization of the former Congo. The museum, which has huge holdings of Congolese and African art and historic objects, reopened last December after an $84 million modernization that aimed to change its colonial image.

The event, which was organized by the event group called Thé Dansant, threatens to undermine the museum’s efforts. The AfricaMuseum was quick to issue an apology. It says it had contacted the organizers to address the problematic dress code, but in hindsight the measure was “insufficient” as several guests showed up wearing stereotypical attire.

Thé Dansant has staged outdoor electronic music events for the past decade, changing their theme depending on their location. The theme of last Sunday’s party was “Afrohouse,” which aimed to celebrate electronic music with African influences. Guests were encouraged to dress up accordingly: “Keep it colorful, African prints, wakanda, la sape!” were suggested by the organizers on Instagram, in reference to the fictional city from the blockbuster film Black Panther and also to colonial-era dandies from the Congo, whose dress was inspired by the French and who were known as “sapeurs,” according to Wikipedia.

While most of the festival-goers avoided cliched stereotypes, social media coverage of the event shows Thé Dansant participants included one dressed as a pith-helmeted explorer, another in blackface, and at least one wearing a an afro wig. Others were seen donning bone necklaces and grass skits. “When the event was announced on Facebook, we noticed that the dress code suggested by Thé Dansant would likely encourage highly clichéd and stereotypical representations of people of African origin,” wrote the museum on Facebook, in response to the criticism.

Café Congo, an artistic collective involved in reflection on Belgian-Congo relations, called the museum out over Facebook: “Explain to me how this sort of event—Thé Dansant—can continue to exist in 2019 at the Africa Museum. Are the management and communications team on Xanax? #NotMyAfricaMuseum #blackface.”

“The Africa Museum misjudged this situation and should have played a greater role in imposing clear requirements and/or conditions in advance,” the institution responded on social media. “We take this incident seriously, and want to apologize for mishandling the situation in such a way that this took place.” The museum stressed it took responsibility for “this lapse in judgment, and are working on an ethical action plan for upcoming events so that this will not happen in the future.”