Twitter has said it fixed a bug in its systems that affected the privacy of more than 93,000 accounts for several months.

The issue affected protected accounts, whose messages are under normal circumstances only visible to “followers” approved by the user, according to Bob Lord, Twitter’s director of information security. In the case of 93,788 of these accounts, non-approved followers were allowed to receive protected tweets via SMS or push notifications, according to Lord.

The bug had been in effect since November 2013, Lord said.

While the number of users is small compared with Twitter’s more than 240 million active users per month, Lord said the company was taking the issue seriously.

“This should not have happened,” Lord said in a blog post on Sunday. “We’ve emailed each of these affected users to let them know about this bug and extend our whole-hearted apologies.”

The unapproved follows have been removed, and Twitter said it has “taken steps” to prevent a similar situation from recurring.