def validate_command_line_parameter_keyword(keyword):
    """
    Validates ``CommandLineParameter``'s `keyword` parameter.
    
    Parameters
    ----------
    keyword : `None` or `str`
        Keyword parameter to validate.
    
    Returns
    -------
    keyword : `None` or `str`
        The validated keyword parameter.
    
    Raises
    ------
    TypeError
        If `keyword` is neither `None` nor `str` instance.
    """
    if keyword is None:
        pass
    elif type(keyword) is str:
        pass
    elif isinstance(keyword, str):
        keyword = str(keyword)
    else:
        raise TypeError(f'`keyword` can be given as `None` or `str` instance, got {keyword.__class__.__name__}.')
    
    return keyword