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 ('7f573450d155fdd68b397107d8a2a7dd', -1, 1438481676, 1438481676, '365b834b', -1, 0, 0)

Line : 187
File : sessions.php