def _fix_dtypes(df):
    """Fix the dtypes of categorial columns.

    Parameters
    ----------
    df : pandas.DataFrame
        data frame to fix

    Returns
    -------
    None
    """
    if u'Strain' in df.columns:
        df[u'Strain'] = df[u'Strain'].astype(str)
    for col in df.columns:
        if df[col].dtype == object:
            df[col] = df[col].astype(str)
