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 ('6873c716f1b31e8c7a9317a9bb882ac3', -1, 1513572400, 1513572400, '36dd5dbb', 5, 0, 0)

Line : 187
File : sessions.php