Access and store value by index in a pointer

I've got an assignment where I have to sum whole numbers up to 100 digits.

They gave me this struct to represent big numbers (I think there are better ways to represent this, but I'm not allowed to modify it):

typedef struct {
  char* string;
  int lengthError;
} BigNumber;

Where string is the number itself and lengthError is the length of the number or an error that is a previously defined enum.

I've also have the implementation of the sum function

BigNumber *sum(BigNumber* num1, BigNumber* num2) {
    BigNumber* result = malloc(sizeof(BigNumber));
    int limit = getLength(num1->lengthError, num2->lengthError);
    result->string = malloc(limit);
    int digitResult;
    int index = limit -1;
    int carry = 0;

    while(index != -1) {
        int d1 = ((int)num1->string[index]) - ((int)'0');
        int d2 = ((int)num2->string[index]) - ((int)'0');
        digitResult = d1 + d2 + carry;

        if (digitResult > 9) {
            digitResult = digitResult - 10;
            carry = 1;
        } else {
            carry = 0;
        }

        itoa(digitResult, &result->string[index], 10); //I think this is the problem
        index--;
    }

    result->string[limit] = '\0';
    printf("result: %s\n", result->string);
    return result;
}

I haven't finished writing that function, I know there are a lot of flaws in it, but the problem is that I can't get to sum 12 + 12. The result I get is 2.

I thought approaching this problem by picking the lasts character of both numbers, transform them into an int and sum them having in mind the carry digit. After I got the result in digitResult I try to convert it to a char and store it in the corresponding position of the result->string pointer

Once it has finished the operation, I add an \0 at the last position of the result->string.

So the question is, how do I make this operation to work as desired? Debugging the code, I noticed that the first time it stores the first result in result->string, following the example above this would be a number 4, it stores trash in that position instead. In the second addition, I store a number 2 correctly and that's the final result I get in when I print the result.