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) VALUES ('ab80b839266df3686c8c789d3b8ad6e6', -1, 1398365636, 1398365636, '3211ae5e', 13, 0)

Line : 158
File : /forum/includes/sessions.php