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 ('6f233e53aec25eda9a5e6df2bfe4d7e1', -1, 1409556441, 1409556441, '36a60a64', 0, 0, 0)

Line : 373
File : sessions.php