For words that have only three letters, there is not any 4th letter. By way of example with the word “add”, there isn't any more letters Later on and therefore needs to be the word "increase". The word “addict” is in the BIP39 wordlist however , you would have applied https://moodjhomedia.com/story2657389/little-known-facts-about-bip39-seed-phrase-list