How do I fix this error with data field 'TR.RNSARPctOSPostTrans'?

My code, which is intended to append new data daily to an already existing csv file named 'Daily API Download Master.csv:

#import packages

import eikon as ek # the Eikon Python wrapper package

import pandas as pd

import numpy as np

import datetime

from datetime import timedelta, date, datetime

from pandas.tseries.offsets import BDay

#connects to Bill's Eikon terminal

ek.set_app_key('xxxx')

#Retreive the data fields from the Eikon API using dynamic dates for a list of RICs

df,e = ek.get_data("lists('Inv Trust List')","TR.RIC")

ric_list = df['Instrument'].tolist()

df, e = ek.get_data(ric_list,

['TR.RNSFilerName',

'TR.RNSAnnouncedDate',

'TR.RNSTransactionType',

'TR.RNSARNumShrsTransacted',

'TR.RNSARPctOSTransacted',

'TR.RNSARTransactionPrice',

'TR.RNSARMktValTransaction',

'TR.RNSARTotShrsPostTrans',

'TR.RNSARPctOSPostTrans'])

end_date = date.today()

start_date = end_date - BDay(1)

end_date_str = datetime.strftime(end_date, "%Y-%m-%d")

start_date_str = datetime.strftime(start_date, "%Y-%m-%d")

df['RNS Announced Date'] = pd.to_datetime(df['RNS Announced Date'])

mask = (df['RNS Announced Date'] > start_date_str) & (df['RNS Announced Date'] <= end_date_str)

df = df.loc[mask]

df.rename(columns={'RNS AR Price (at Transaction) - £': 'RNS AR Price (at Transaction) GBP',

'RNS AR Market Value of Transaction - £': 'RNS AR Market Value of Transaction - GBP'},

inplace=True)

#append new csv to Master

df.to_csv('Daily API Download Master.csv', mode='a', header=False)

Backend error. 400 Bad Request

---------------------------------------------------------------------------
EikonError                                Traceback (most recent call last)
<ipython-input-16-f4c48e9e9e88> in <module>()
     22                      'TR.RNSARMktValTransaction',
     23                      'TR.RNSARTotShrsPostTrans',
---> 24                      'TR.RNSARPctOSPostTrans'])
     25      26 end_date = date.today()

~\AppData\Local\Continuum\anaconda3\lib\site-packages\eikon\data_grid.py in get_data(instruments, fields, parameters, field_name, raw_output, debug)
    184         payload = {'requests': [payload]}
    185 --> 186     result = eikon.json_requests.send_json_request(_endpoint, payload, debug=debug)
    187     188     if result.get('responses'):

~\AppData\Local\Continuum\anaconda3\lib\site-packages\eikon\json_requests.py in send_json_request(entity, payload, debug)
    116                         ticket = _check_ticket_async(result)
    117 --> 118                 _check_server_error(result)
    119                 return result
    120             else:

~\AppData\Local\Continuum\anaconda3\lib\site-packages\eikon\json_requests.py in _check_server_error(server_response)
    192             status, reason_phrase, version, content, headers = error_message.split(',')[:5]
    193         logger.error(error_message)
--> 194         raise EikonError(int(server_response['ErrorCode']), error_message)
    195     196     # check DataGrid response (server response is JSON and it can contain error + transactionId keys)

EikonError: Error code 400 | Backend error. 400 Bad Request