In line 406 of flex2csv.py, you split an ID into sentence number (before period) and phrase number (after period). This fails if the sentence number had not been retrieved.

Be tolerant with records without sentence numbers. Users might still want to convert them.

As a quick fix, I used

    if "." in rec.get("Sentence_Number", "noperiod"):
        rec["Sentence_Number"], rec["Phrase_Number"] = rec["Sentence_Number"].split(".")
    return rec

but try/catch or checking for "Sentence_Number" in rec should also work