On Monday, The Anne Frank Center for Mutual Respect found itself on the wrong side of Facebook’s moderation system. The group had attempted to share a new study on Holocaust awareness in America — but while the post went through fine on Twitter, it was abruptly blocked by Facebook. The Center wrote back demanding an explanation, but received nothing in response.

After two days of silence from Facebook, the company publicly called out Facebook for the misstep. “You removed our post promoting the need for Holocaust Education for apparently violating community standards,” the Center wrote on Twitter Wednesday morning. “You haven’t given us a reason.”

The image attached to the post depicts a group of nude, emaciated children, which seems to have triggered Facebook’s nudity policy, although the age and poor quality of the image obscures much of the scene.

Facebook restored the post a few hours after the Frank Center’s public statement, and has apologized for the error. “As our Community Standards explain, we don’t allow people to post nude images of children on Facebook,” a company representative explained to The Verge. “We recognize that the image shared by the Anne Frank Center is historically significant and important, and was restored on this basis.”

Reached for comment, the Anne Frank Center’s Alexandra Devitt struck a troubling contrast with Facebook’s lax approach to pages that deny the Holocaust. “While Facebook removes the AFC’s post promoting the need to educate on the past, it continues to allow pages and posts that directly deny the reality of the deaths of more than six million people,” Devitt told The Verge. “If Facebook is serious about its community standards it should start tackling Holocaust denial and not the organizations who are trying to educate people on discrimination, facts, and history.”

A small organization with few staffers and an ambiguous connection to the Frank family, the Center has gained prominence in recent years for its willingness to take on the Trump administration and other right-wing groups. That activism has included direct confrontations with Facebook, with one campaign gaining more than 180,000 signatures for a Change.org petition demanding the company take down pages promoting Holocaust denial.

This isn’t the first time Facebook has struggled with historically important images involving nudity. In 2016, the platform blocked an iconic news photo from the Vietnam War, showing a naked nine-year-old girl fleeing a napalm strike. Facebook restored the photo after a public outcry, citing its historical importance.