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 ('ddadd46502e03e863384f9dbc5abb2aa', -1, 1371623281, 1371623281, '6b140741', -1, 0, 0)

Line : 173
File : sessions.php