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, session_admin) VALUES ('613261b08a68e96d0e7c301e5019aa97', -1, 1406270840, 1406270840, '36cd77a3', 35, 0, 0)

Line : 180
File : sessions.php