For words that have only 3 letters, there's no 4th letter. One example is with the word “insert”, there isn't any additional letters Later on and therefore has to be the word "include". The word “addict” is while in the BIP39 wordlist however you would've applied “addi” for “addict”. A https://socialstrategie.com/story4421253/fascination-about-invalid-bip39-phrase