(Reuters) - Facebook Inc FB.O acknowledged a flaw in its Messenger Kids app, weeks after two U.S. senators raised privacy concerns about the application, and said that it spoke to the U.S. Federal Trade Commission about the matter.

FILE PHOTO - Attendees walk past a Facebook logo during Facebook Inc's F8 developers conference in San Jose, California, U.S., April 30, 2019. REUTERS/Stephen Lam

“We are in regular contact with the FTC on many issues and products, including Messenger Kids,” Facebook Vice President Kevin Martin wrote in a letter to two Democrats, which is seen by Reuters. He described the flaw as a “technical error”.

The letter dated Aug. 27 was sent to Democratic Senators Ed Markey of Massachusetts and Richard Blumenthal of Connecticut.

“Based on our review, we have determined that the technical error you have inquired above arose in October 2018. The fix we implemented is designed to prevent the issue from happening again,” Facebook said in the letter.

The senators said on Wednesday they were disappointed by Facebook’s approach on the matter.

“We are particularly disappointed that Facebook did not commit to undertaking a comprehensive review of Messenger Kids to identify additional bugs or privacy issues,” Markey and Blumenthal said in response to Facebook’s letter.

The senators had written to Facebook on Aug. 6 questioning whether there was a “worrying pattern” of poor privacy protection for children using the Messenger Kids app and sought transparency.

The senators were “disturbed” to learn that the application allowed thousands of children to join group chats in which not all members of the group were approved by their parents, they said in the letter to Facebook Chief Executive Officer Mark Zuckerberg.

In July, Facebook agreed to pay a record $5 billion fine to resolve an FTC probe into its privacy practices and will boost safeguards on user data.

FTC did not immediately respond to a Reuters request for comment outside regular business hours.