Don't understand why there I get "TypeError: 'str' object cannot be interpreted as an integer" in my code

from Yatzy import dice


class Hand(list):
    def __init__(self, size=0, die_class=None, *args, **kwargs):
        if not die_class:
            raise ValueError("You must provide a die class!")
        super().__init__()

        for _ in range(size):
            self.append(die_class())

    def __len__(self):
        str_num = str(super().__len__())
        return "There are {} die in this hand".format(str_num)


hand = Hand(size=5, die_class=dice.D6)

print(hand)
print(len(hand))

When I try to run this code it gives me this error:

TypeError: 'str' object cannot be interpreted as an integer

I've tried analyzing the problem with is instance() for example but I've not come to any conclusions where the problem lies.

I know that print() can take a string as an argument. I also know that if I check the instance of super()__len__() I get back True when I check for an int. If I then do str(super()__len__()) and check what type of instance it is, I get back True for a string and False for an int. When I then return it to be used in print() I get this weird error.

What is happening here?

This is just something extra I wanted to add to keep practicing with overwriting inbuilt methods but I would really like to learn from this.