Doesn't work for a large file?

[mauser8004]

LogCheck.py

Traceback (most recent call last):
File "LogCheck.py.2", line 111, in
analyze_weblog(weblogfileName)
File "LogCheck.py.2", line 40, in analyze_weblog
user_agent = row[apachelogsfields.index('user_agent')]
IndexError: list index out of range

Logfile :
wc -l /var/srvlog/2021.04/access.log

400053 /var/srvlog/2021.04/doktori/access.log

It works with a smaller size:
wc -l /var/srvlog/2021.03/access.log
229303 /var/srvlog/2021.03/access.log