TypeError: invalid 'in' operand obj

嘗試在程式去訪問遠程的Web API，它在運行時，出現異常： 

$(function () {
            $.ajax({
                type: "GET",
                url: "http://localhost:9001/api/size",                
                dataType: "json",
                contentType: "application/json; charset=utf-8",
                success: function (data) { 
                    data = $.parseJSON(data);
                    $.each(data, function (index, item) {
                        //...
                    });                  

                },
                error: function (jqXHR, textStatus, errorThrown) {
                    alert(errorThrown);
                }
            });
        });