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, is_robot, session_admin) VALUES ('6b1f7e85eb970e7c94eb14e006651ba6', -1, 1455100934, 1455100934, '36c4d92b', 9, 0, '0', 0)

Line : 174
File : sessions.php