phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1062 Duplicate entry '-1' for key 'session_user_id'

INSERT INTO phpbb_sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in, session_admin) VALUES ('8ab9337430f72ef5b5b2f373f02c31f8', -1, 1416873952, 1416873952, '36a635a9', 6, 0, 0)

Line : 188
File : sessions.php