jquery.min 500 internal error which code causes this?

hi there i'm getting 500 internal server error when im pressing this button which does this :

$(".btn-email").on('click', function() {
    swal('Waiting','Please wait, sending email now','info');
    $.getJSON("/api/addvoter",{"email":$(".voter-email").val(),"vote_id":$(".vote-id").val()},function(result){
        if(result.success == 1){
            swal("Good job!", "Send an email to the voter successfully!", "success");
            $(".voter-email").val("");
        }else{
            sweetAlert("Oops...", result.content, "error");
        }
    });

});

error image

i'm using open-source blockvote's system which is here

what could be causing this error?