Connection error: 0 - Error parsing instrument tokens.

I am getting the below error after on_connect is executed:
"Connection error: 0 - Error parsing instrument tokens."

below are the lines in the on_connect procedure:
ws.set_mode(ws.MODE_LTP, instrument_token)

I have used variable instrument_token to store nifty 18 sep future through a congif.ini file which has the following last line:
instrument_token = [10991874]

This was working fine till yesterday. Let me know if anyone faced similar issue.

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