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 ('6b1e0c44f13c4334030c4e7cc96d7bbc', -1, 1398063867, 1398063867, '6b1646d7', 5, 0)

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