I have a PHP script that breaks if a variable is not populated and it isn't added to the database, but jQuery handles this as a success and I get this error:

TypeError: Result of expression 'data' [null] is not an object.

Here's the jQuery script:

$.ajax({
    type: "POST",
    url: "/clase/do-add",
    data: $("#adauga").serialize(),
    dataType: "json",
    error: function (xhr, textStatus, errorThrown) { 
        alert('Try again.');
    },
    success: function(data) {
        var dlHTML = '<dl id="' + data.id + '"> [too long] </dl>';
        $('form#adauga').after(dlHTML);
        $('#main dl:first').hide().fadeIn();

        adaugaClasaSubmit.removeAttr('disabled');
        adaugaClasa.removeAttr('readonly');
        adaugaClasa.val("").focus();
    }
});