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 ('62e061d5ccf78b6d1600f696b4e67b91', -1, 1408672600, 1408672600, '3652043a', -1, 0, 0)

Line : 187
File : sessions.php