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 ('6a12aeec93dcff587ef61337c568e4e6', -1, 1398091194, 1398091194, '3211a2ae', 4, 0, 0)

Line : 187
File : sessions.php