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 ('d8b3077068ecd4f55b93d45672b0edc1', -1, 1488300563, 1488300563, '36cd6a8a', 0, 0, 0)

Line : 187
File : sessions.php