Gives me error, my local editor says its ok

My Code:

def index_power(array, n):
    """
        Find Nth power of the element with index N.
    """

    if len(array) > n:
        return (array[n] ** n)
    else:
        return (-1)


if __name__ == '__main__':
    #These "asserts" using only for self-checking and not necessary for auto-testing
    assert index_power([1, 2, 3, 4], 2) == 9, "Square"
    assert index_power([1, 3, 10, 100], 3) == 1000000, "Cube"
    assert index_power([0, 1], 0) == 1, "Zero power"
    assert index_power([1, 2], 3) == -1, "IndexError"