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 ('a9ba7fa2cdb8719b13a3f9dfed4d0d76', -1, 1540074254, 1540074254, '36e0387e', 7, 0, 0)

Line : 187
File : sessions.php