def human_readable(millis): 
    """ 
    Take a number of milliseconds and turn it into a string with the format
    "min:sec.millis".
    """
    if not isinstance(millis, int):
        raise TypeError('millis must be a positive integer')

    if millis < 0:
        raise ValueError('millis must be a positive integer')

    seconds, ms = divmod(millis, 1000)
    minutes, seconds = divmod(seconds, 60)

    return '{}:{}.{}'.format(minutes, seconds, int(ms))