Why isn't this code passing the Hand challenge?

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)

from dice import D20

class Hand(list):
    def __init__(self, size=0, die_class=None):
        super().__init__()  # I know I could check if die_class is truthy before this line, but I think the code should run fine for the purposes of the challenge.
        for _ in range(size):
            self.append(die_class())

    @classmethod
    def roll(cls, size, die_class = D20):
        return cls(size, die_class)                        


    @property
    def total(self):
        return sum(self)