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 ('0d686bc9c1bc3c3d015f2e5b6c5c4aff', -1, 1398365633, 1398365633, '3211ae5e', 1, 0)

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