phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1062 Duplicate entry '-1' for key 'session_user_id'

INSERT INTO sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in, session_admin) VALUES ('a9fa5ba76b11519befe93a1c0cb0c794', -1, 1461909803, 1461909803, '365a9d23', -9, 0, 0)

Line : 187
File : sessions.php