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 ('6476c343185d4cb7b65e9892bae54be0', -1, 1488300359, 1488300359, '36cd6a8a', -1, 0, 0)

Line : 187
File : sessions.php