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 ('fde93f7bd1bd0d72b7a66cb5f7a1dbe9', -1, 1414827968, 1414827968, '36577b63', -9, 0, 0)

Line : 187
File : sessions.php