For words that only have 3 letters, there is no 4th letter. For instance with the word “add”, there are no more letters Later on and so have to be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would've applied “addi” for “addict”. BIP39 and https://pr6bookmark.com/story19189337/invalid-bip39-phrase-things-to-know-before-you-buy