1366 - Incorrect integer value: '' for column 'customer_id' at row 1

insert into whos_online (customer_id, full_name, session_id, ip_address, time_entry, time_last_click, last_page_url) values ('', 'Guest', 'vudtjfi8mu120g9no8p5jj9380', '216.73.216.9', '1758033748', '1758033748', '/index.php?XTCsid=c407ede434192d8d024c7c2f06b2be8c&language=de')

[XT SQL Error]