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 ('6b7cb3008bd652b33c94fd619561acc4', -1, 1406175680, 1406175680, '36e2b4df', 83, 0, 0)

Line : 180
File : sessions.php