Using this line of code:
df, err = ek.get_data(['ESU0'], ['TR.SETTLEMENTPRICE.DATE', 'TR.OPENPRICE', 'TR.HIGHPRICE', 'TR.LOWPRICE', 'TR.SETTLEMENTPRICE', 'TR.ACCUMULATEDVOLUME', 'TR.OPENINTEREST'], {'SDate': "01/01/2006", 'EDate': "12/31/2020", 'Frq': 'd'})
returns many NaN entries for Open, High and Low on dates where there is a Settlement price and volume & OI. Settlement price for most recent traded day (18 Sep 2020) is also NaN.