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', 'g1esm489u540esl3fnh6k4fie5', '100.25.214.89', '1561366850', '1561366850', '/shopping_cart.php?language=de')

[XT SQL Error]