不允许使用关键字符

Question

I am posting the form using jQuery serialize in my Codeigniter application. But I am getting a response like disallowed key characters. The posted URL is like below.

candidate_name=1000026&candidate_name=&candidate_name=&candidate_name=&candidate_name=&candidate_name=&candidate_name=&candidate_name=&candidate_name=&candidate_name=&candidate_name=&board=&math_min_marks=&math_max_marks=&math_marks_secured=&math_marks_percent=&physics_min_marks=&physics_max_marks=&physics_marks_secured=&physics_marks_percent=&optional_subject=&optional_min_marks=&optional_max_marks=&optional_marks_secured=&optional_marks_percent=&mobileNum=&email=&agree1=agree1&agree2=agree2&agree3=agree3

And the jQuery post is,

var datastring = $("#register").serialize();

$.ajax({
    type: "POST",
    url: "<?php echo base_url('register/savedraft'); ?>",
    data: JSON.stringify(datastring),
    dataType:"json",
    success: function (result) {
        if(result){
            alert(result)
        }
    }
});