def prime_factor(x):
    """ Return the prime factor of input x

    Parameters
    ----------
    x (int): input natural number > 1

    Return
    ------
    sorted_pf (set): list of prime factor of x """

    if not x:
        raise ValueError("Empty input.")

    if not isinstance(x, int):
        raise ValueError("Input value must be integer.")

    if x < 2:
        return None

    pf = set()
    while x > 1:
        for i in range(2, x + 1):
            if x % i == 0:
                x = int(x / i)
                pf.add(i)
                break
    sorted_pf = sorted(pf)

    return sorted_pf