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 ('f3a3e8b0b83205ac05249b065d681adc', -1, 1438078820, 1438078820, '369051eb', -1, 0, 0)

Line : 187
File : sessions.php