Oшибка list index out of range. После версии 3.9 код работает некорректно

alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]


direction = input("type 'encode' to encrypt, type 'decode' to decrypt:\n")

text  = input("write your message:\n").lower()

shift = int(input("type the shift number:\n"))

def encrypt(plain_text,shift_amount):
    cipher_text = ""
    for letter in alphabet:
        position = alphabet.index(letter)
        new_position = position + shift_amount
        new_letter = alphabet[new_position] # проблема тут ! 
        cipher_text += new_letter
    print(f"the encoded text is {cipher_text}")

encrypt(plain_text=text,shift_amount=shift)