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 ('6c1ac8d9c8be5dffc957169fb70fc76f', -1, 1455224936, 1455224936, '1716b61d', 6, 0, '0', 0)

Line : 174
File : sessions.php