The following code attempts to remove k digits from a number string to get the smallest number. Which line contains a subtle bug that causes incorrect results on inputs with leading zeros?

The following code attempts to remove k digits from a number string to get the smallest number. Which line contains a subtle bug that causes incorrect results on inputs with leading zeros?

def removeKdigits(num: str, k: int) -> str:
    builder = []
    for digit in num:
        while k > 0 and builder and builder[-1] > digit:
            builder.pop()
            k -= 1
        builder.append(digit)
    while k > 0:
        builder.pop()
        k -= 1
    result = ''.join(builder)  # Bug here
    return result if result else '0'