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 ('186985364d637c5e8f627d87d1ea2f0b', -1, 1416974063, 1416974063, '36502b5a', 8, 0, 0)

Line : 191
File : sessions.php