Facebook said that a “technical error” allowed thousands of children using its Messenger Kids app to join chats with unauthorized users.

Messenger Kids is a Facebook app specifically built for children between 6 to-12 years old. The app has been designed to make sure that the children under the age of 13 do not interact with people who have not been approved by their parents.

However, a design flaw allowed children to enter group chats with strangers — undermining the entire premise of the app.

The company has notified thousands of parents as soon as the flaw was discovered, but has not said how long the flaw was active. It is also unclear how many complaints were received prior to the disclosure.

Consumer groups filed a complaint in 2018 arguing the Messenger Kids capp violates children’s privacy and does not meet the minimum standard set by the Children’s Online Privacy Protection Act (COPPA) because it collects kid’s personal information without obtaining verifiable parental consent.