WP_Error Invalid term ID appears when WordPress tries to work with a term whose identifier is missing or malformed. The error originates from core functions that expect a numeric term_id but receive 0, false, or a non‑existent value.
Root causes include a corrupted wp_terms auto_increment, mismatched taxonomy slugs, boolean values passed as IDs, and orphaned rows in term tables. When the system receives an invalid ID it returns a WP_Error object with the code invalid_term_id.