I was working in a social project and working with captcha to protect registration form spammers. I had use captcha helper several time in my projects but this time I found an unexpected problem. 

at the very first, I though it might be database problem. I though I might allow the NULL value to the field. But I found table structure is ok. 

Then I remember the captcha_time will be provided by Captcha helper. So I have checked the controller and autoload file, helper function was called. So there were no problem at all. 

Then I checked my local server. Program is running fine there. Problem is in online. finally I found it. I forgot to create a captcha folder in my server where the captcha image will be created. So the error occurred.