For an unknown period of time, Facebook let advertisers target users based on keyword combinations like “Jew hater” and “How to burn jews.”

In fact, it wasn’t until ProPublica brought the anti-Semitic categories to Facebook’s attention, this week, that the company removed them from its ad-targeting menu.

In response, Facebook said its ad-buying system is not perfect.

“There are times where content is surfaced on our platform that violates our standards,” Rob Leathern, product management director at Facebook, said in a prepared statement.

“We know we have more work to do, so we’re also building new guard rails in our product and review processes to prevent other issues like this from happening in the future,” Leathern added.

In general, Leathern noted that Facebook’s community standards “strictly prohibit attacking people based on their protected characteristics, including religion, and we prohibit advertisers from discriminating against people, based on religion and other attributes.”

While accepting some blame for carelessly catering to anti-Semites, Facebook does take issue with ProPublica attributing the hateful ad categories to an “algorithm.”

The categories in question were self-reported, based on how users filled out their profiles, according to the social giant.

In other words, Facebook is saying that users filling out their profiles may have added descriptions like “Jew hater,” which then appeared to advertisers as potential categories.

In all, ProPublica reports that roughly 2,300 people expressed interest in the anti-Semitic categories.

This is not the first time that ProPublica has caught flaws in Facebook’s ad system.

Late last year, the nonprofit organization reported  that Facebook was making it possible for advertisers to prevent their ads from being shown to users who belonged to certain “ethnic affinity” groups.

That ProPublica report sparked outrage among lawmakers and activists, which led to a potential class-action civil rights lawsuit.

In response, Facebook promised to prohibit marketers from using a race-based targeting option for ads offering housing, employment or credit.