For words that have only 3 letters, there's no 4th letter. By way of example with the word “add”, there aren't any far more letters Later on and so needs to be the word "include". The word “addict” is while in the BIP39 wordlist but you might have used “addi” https://push2bookmark.com/story19299129/not-known-factual-statements-about-bip39-explained