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', 'un1mhanop9ud905tu1mtvnmig5', '216.73.216.80', '1751317836', '1751317836', '/index.php?language=de&cat=c4_Beer-Mugs.html&XTCsid=e990f3916c880610adc34c2fa7d30f83')

[XT SQL Error]