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 ('fd964cfbbc0bed14e6d1d30e0354cc7a', -1, 1467353614, 1467353614, '36c655c4', 0, 0, 0)

Line : 201
File : sessions.php