About is_ajax_request

Hi! I'm new in Codeigniter, and I'm making this question because I didn't find anything about that.

So, I did an add comment system using Ajax in the view, that calls a function in my Post controller. I saw that if I went on "localhost/controller/function" I got a mysql error because some fields were "NULL". So I inserted 

PHP Code:

$this->input->is_ajax_request() 


but I was asking myself: can I bypass this check in some way? Is this secure? Can be spoofed? 

If you have some suggestion to protect my function I'll learn something new ! 
Best regards!