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 ('987c2bb23321896359a4d1b42ba64ccd', -1, 1416564939, 1416564939, '36a7b1b4', 21, 0, 0)

Line : 191
File : sessions.php