Why Json is null in this situation?

I'm trying to return a JSON encoded array from a PHP page via ajax. here's my code from the calling page

---

$('#test_load').click(function(){

        $.ajax({

            type:"POST",

            url:'/actions/admin_load.php',      

            dataType: 'json',

            asynch:false,

            data:"action=4",

            success: function(json) {

                alert("we're back");

                alert(json.a);

            }

        });

    });

---

here's my php code

//return json array

    case "4" :

        $str2 =  "HELLO";

        $arr = array ('a'=>1,'b'=>2,'c'=>3,'d'=>4,'e'=>5);

            $str = json_encode($arr);

        echo $str;



    break;

---

I keep getting "json is null". Any suggestions would be much appreciated