def parse_keys(keys):
    """
    Examples:
    >>> parse_keys('accesskey:secretkey')
    ('accesskey', 'secretkey')
    >>> parse_keys('')
    (None, None)
    >>> parse_keys(None)
    (None, None)
    >>> parse_keys('accesskey') # Missing secret key
    Traceback (most recent call last):
        ...
    ValueError: need more than 1 value to unpack
    """
    if keys:
        access_id, secret_key = keys.split(':')
        return access_id, secret_key
    else:
        return None, None