Can't apply for dynamic length of captcha

[hoangatpaygate]

I have got an issue when run the captcha_records.py, the problem is that my captcha length is 6 or 7 characters, so I set CHARS_NUM = 7, when run the captcha_records.py, it shows an error
"IndexError: index 7 is out of bounds for axis 0 with size 7"
How can I apply for dynamic length of captcha?

Thank you very much!