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 ('86c3872eda7e334445d4de844c7331fa', -1, 1474781085, 1474781085, '36f2e0fa', -1, 0, 0)

Line : 187
File : sessions.php