WordPress generates a WP_Error object with the code “invalid_post” when a function receives a post identifier that does not match any record in the wp_posts table. The error stops the operation and returns false or a WP_Error instance.
The condition appears when the identifier is missing, zero, non‑numeric, mismatched after a migration, filtered out by custom code, or when the current user lacks the edit capability. Database corruption or an incorrect auto‑increment value can also produce the same result.