Stuck in hands.py code challenge, everything seems in order

import random

class Die:
    def __init__(self, sides=2):
        if sides < 2:
            raise ValueError("Can't have fewer than two sides")
        self.sides = sides
        self.value = random.randint(1, sides)
    def __int__(self):
        return self.value
    def __add__(self, other):
        return int(self) + other
    def __radd__(self, other):
        return self + other

class D20(Die):
    def __init__(self):
        super().__init__(sides = 20)

class Hand(list):
    def __init__(self, lst = []):
        self.extend(lst)
    @classmethod
    def roll(cls, times):
        lst = []
        for _ in range(times):
            lst.append(D20())
        return cls(lst)
    @property
    def total(self):
        sum = 0
        for d in self:
            sum += d
        return sum