jquery can't select a simple li

could someone explain to me why in a page with this code

<ul id="test">
<li>bla</li>
</ul>

if do

alert($("#test").html());

I get the right content but if I do one the followings

alert($("#test li").html()); 
alert($("#test > li").html());
alert($("#test li:first-child").html());

I get an error in firebug saying

$("#test li").html is not a function

EDIT: jquery is included in fact this thing doesn't make sense, could it be because this code is launched in a function in an ajax "success"?

REST OF THE CODE //right now it doesn't do anything but if I can't select the li i can't continue

$("#add_entry_form").submit(

                        function(){

                            //get data
                            var title = $(this).find("#title").val();
                    开发者_开发知识库        var body = $("#editor").val();
                            var type = $(this).find("#type").val();


                            $.ajax({
                                  type: 'POST',
                                  url: "add_entry.php",
                                  dataType: "json",
                                  data: "title="+encodeURIComponent(title)+"&body="+encodeURIComponent(body)+"&type="+type,
                                  success: function(response){                                     


                                           alert($("#test li").html());



                                  }
                            });


                        return false;

                        }
        )