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 ('39400b1db0bf0469f39a9f9fa173f2e9', -1, 1413926885, 1413926885, '36517bd8', 0, 0, 0)

Line : 373
File : sessions.php