Leetcode: Python3 - Integer to English solution - working locally but giving error on submission

CODE

class Solution:
    def numberToWords(self, num: int) -> str:
        string = str(num)
        rev_str = string[::-1]
        str_list = []

        i = 0
        while i < len(rev_str):
            if rev_str[i] == "0":
                str_list.append("")
            elif rev_str[i] == "1":
                str_list.append("One")
            elif rev_str[i] == "2":
                str_list.append("Two")
            elif rev_str[i] == "3":
                str_list.append("Three")
            elif rev_str[i] == "4":
                str_list.append("Four")
            elif rev_str[i] == "5":
                str_list.append("Five")
            elif rev_str[i] == "6":
                str_list.append("Six")
            elif rev_str[i] == "7":
                str_list.append("Seven")
            elif rev_str[i] == "8":
                str_list.append("Eight")
            elif rev_str[i] == "9":
                str_list.append("Nine")
            i += 1

        # print(str_list)
        i = 1
        while i < len(rev_str):
            if rev_str[i] == "0":
                str_list[i] = ""
            elif rev_str[i] == "1":
                if rev_str[i-1] == "0":
                    str_list[i] = "Ten"
                elif rev_str[i-1] == "1":
                    str_list[i] = "Eleven"
                elif rev_str[i-1] == "2":
                    str_list[i] = "Twelve"
                elif rev_str[i-1] == "3":
                    str_list[i] = "Thirteen"
                elif rev_str[i-1] == "4":
                    str_list[i] = "Fourteen"
                elif rev_str[i-1] == "5":
                    str_list[i] = "Fifteen"
                elif rev_str[i-1] == "6":
                    str_list[i] = "Sixteen"
                elif rev_str[i-1] == "7":
                    str_list[i] = "Seventeen"
                elif rev_str[i-1] == "8":
                    str_list[i] = "Eighteen"
                elif rev_str[i-1] == "9":
                    str_list[i] = "Nineteen"
                str_list[i-1] = ""
            elif rev_str[i] == "2":
                str_list[i] = "Twenty"
            elif rev_str[i] == "3":
                str_list[i] = "Thirty"
            elif rev_str[i] == "4":
                str_list[i] = "Forty"
            elif rev_str[i] == "5":
                str_list[i] = "Fifty"
            elif rev_str[i] == "6":
                str_list[i] = "Sixty"
            elif rev_str[i] == "7":
                str_list[i] = "Seventy"
            elif rev_str[i] == "8":
                str_list[i] = "Eighty"
            elif rev_str[i] == "9":
                str_list[i] = "Ninety"
            i += 3

        # print(str_list)
        i = 3
        while i < len(str_list):
            if num > 99:
                if i % 3 == 0:
                    str_list[i-1] = str(str_list[i-1]) + " Hundred"
            i += 3

        # print(str_list)
        i = 0
        while i < len(str_list):
            if i == 3:
                str_list[i] = str(str_list[i]) + " Thousand"
            elif i == 6:
                str_list[i] = str(str_list[i]) + " Million"
            elif i == 9:
                str_list[i] = str(str_list[i]) + " Billion"
            i += 1

        # print(str_list)
        xray = []
        for i in str_list:
            if i != "":
                xray.append(i)

        # print(x)
        xray = xray[::-1]
        print(xray)
        
        if num < 1000: xray[0] += " Hundred"

        answer = " ".join(xray)

        # print("["+answer+"]")
        answer = answer.replace("  ", " ")
        if answer == "":
            return "Zero"

        # return "["+answer+"]"
        return answer

Code explanation: basically the code is supposed to turn integers into English so:

Example 1:
Input: num = 123
Output: "One Hundred Twenty Three"

Example 2:
Input: num = 12345
Output: "Twelve Thousand Three Hundred Forty Five"

Example 3:
Input: num = 1234567
Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

the way i approached this solution was to first turn all the numbers into a list of words, the reverse the list, then turn every 3rd-1 index in to a teen word and then put either thousand, million, billion every 3rd index.

Error on LeetCode:

IndexError: list index out of range
    if num < 1000: xray[0] += " Hundred"
Line 105 in numberToWords (Solution.py)
    ret = Solution().numberToWords(param_1)
Line 134 in _driver (Solution.py)
    _driver()
Line 145 in <module> (Solution.py)

Locally when I run something like:

if __name__ == "__main__":
    s = Solution()
    print(s.numberToWords(123))

i get the answer

['One', 'Twenty', 'Three']
One Hundred Twenty Three

which is expected, but i get the error above on leetcode.