def fill_missing(fields, row):
    """
    Fills missing fields with nulls.
    Args:
        fields (list<str>): list of column names that should be filled.
        row (dict): row object to fill.
    Returns:
        Dict of new row with missing columns filled with nulls.
    """
    for col in fields:
        if col not in row:
            row[col] = None
    return row