bluejellybean

    if n == 0:

        return 0

    else:

        return n % 10 + sum_of_digits(n // 10)

    if n < 0:  # Added base case check for negative numbers

        return None

    elif not isinstance(n, int):  # Check type of argument passed in

        raise TypeError("Input must be an integer")

    elif n == 0:

        return 0

    else:

        return n % 10 + sum_of_digits(n // 10)

    def test_sum_of_digits_positive(self):

        self.assertEqual(sum_of_digits(123), 6)



    def test_sum_of_digits_negative(self):

        self.assertEqual(sum_of_digits(-123), None)


    def test_sum_of_digits_type(self):

        with self.assertRaises(TypeError):

            sum_of_digits('abc')