Identify the error in this Order class method that calculates total cost:

Identify the error in this Order class method that calculates total cost:

class Order:
    def __init__(self, cart):
        self.cart = cart
        self.total = 0
    def calculate_total(self):
        for product, qty in self.cart.items():
            self.total += product.price * qty
        return self.total