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 ('b64e2fa39398763b8c65313b9a39a8bc', -1, 1481141786, 1481141786, '3693d375', -1, 0, 0)

Line : 187
File : sessions.php