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 ('cb5cd5988099f74b1e2f2dfcd9bf9018', -1, 1406238054, 1406238054, '36eb109f', 81, 0, 0)

Line : 180
File : sessions.php