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 ('ad7a0519c18b168de79e021f38b6c607', -1, 1397999895, 1397999895, '36eb24a4', 3, 0)

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