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', '8crjttk0m9bmp6gi6lspskms57', '216.73.216.63', '1774112545', '1774112545', '/index.php?XTCsid=ca28ab8f46bb6e791b42fe9b92719761&language=de')

[XT SQL Error]