import re


def is_mixed_case(s):
    """
    >>> is_mixed_case('HiThere')
    True

    >>> is_mixed_case('Hi There')
    True

    >>> is_mixed_case('hi there')
    False

    >>> is_mixed_case('h')
    False

    >>> is_mixed_case('H')
    False

    >>> is_mixed_case(None)
    False

    """
    if not isinstance(s, (str,)):
        return False

    mo_lo = re.search(r'[a-z]', s)
    mo_hi = re.search(r'[A-Z]', s)

    if ((mo_lo is None) or (mo_hi is None)):
        return False

    return True