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 ('a21902a7ddbcb0455cf482c682dd5d6e', -1, 1455468633, 1455468633, '36c4ef1c', -1, 0, 0)

Line : 187
File : sessions.php