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', 'nph2r3lh5p099uko3aeb375j83', '216.73.216.147', '1751839930', '1751839930', '/product_info.php?language=de&info=p15_25-balloons.html&XTCsid=a6fef95dcbdcf765a3c6baf6e1fe0e25')

[XT SQL Error]