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 ('26f397b3b31baade13dddd980a5ed3cd', -1, 1409709997, 1409709997, '3210a9dc', 1, 0)

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