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 ('db79d8cde3d2700c1016c2ddbe0185db', -1, 1406207023, 1406207023, '36c53d58', 37, 0, 0)

Line : 180
File : sessions.php