String index out of range - Python

I am making an encrypter in Python and it keeps sending me a "string index out of range" message.

input = input("").lower()
letters =["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"]
message = []
encryption = []
letter = 0
code = 1
wordletter = 0
word = "baa"

for i in range(len(input)):

    code = 1

    wordletter += 1

    if wordletter > len(word):
        wordletter = 0

    code = letters.index(word[wordletter])

    if input[i] in letters:
        letter = letters.index(input[i])
        letter += code
        message.append(letter)
    else:
        message.append(input[i])

for i in range(len(message)):
    encryption.append(letters[message[i]])

It says the error is on line 19 - like this.

Traceback (most recent call last):
  File "//x17/StudentHome/2023/a.nachmany16/Encrypter.py", line 19, in <module>
    code = letters.index(word[wordletter])
IndexError: string index out of range
>>> 

Can someone help me?