For words that have only three letters, there isn't a 4th letter. For instance with the word “insert”, there won't be any more letters Later on and therefore should be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would have utilised “addi” for “addict”. - https://bookmarkja.com/story22028885/a-review-of-bip39-passphrase