IndexError: string index out of range. Newbie at coding

Question

I'm pretty beginner to Python so I get the error

Traceback (most recent call last):

File "C:/Users/spchee/Documents/cc.py", line 43, in c("helloz", 2) File "C:/Users/spchee/Documents/cc.py", line 35, in c add_to_string(n) File "C:/Users/spchee/Documents/cc.py", line 5, in add_to_string if message[i].titlecase():

IndexError: string index out of range

My current code at the moment is:

import math
message = ""
i=0
def add_to_string(modifier):
    if message[i].titlecase():
        encrypted = encrypted + string[modifier].upper()
    else:
        encrypted = encrypted +string[modifier]

def c(message,key):



    string = "0abcdefghijklmnopqrstuvwxyz0"
    encrypted = ""
    legnth_message = len(message)
    i = 0


    for i in range (legnth_message-1):

        n = 0
        x = 0

        while message[i] != string[x].lower():
            x+=1;

        while string[int(x)+int(n)] != "0":
            n += math.copysign(1, key)

        if abs(key)<abs(n):
            n = (abs(n)-abs(key))*math.copysign(1,key)

            if math.copysign(1,key) == 1:
                add_to_string(n)
            else:
                add_to_string(27+n)

        else:
            add_to_string(x+key)


c("helloz", 2)

The code should function as a Caesar Cipher. Thanks in advance