knockout sortable with computed observable not working

jsfiddle example. Like the title says I am trying to use a computed observable along with rniemeyer knockout sortable example. I keep getting

the write method needs to be implemented

This error is viewable in the developer console.

I have a write method implement on my ko.computed but it still errors out. What I am I doing wrong?

html and javascript below

<div id="main">
    <h3>Tasks</h3>
    <div class="container" data-bind="sortable: tasks">
        <div class="item">
            <span data-bind="visible: !$root.isTaskSelected($data)">
                <a href="#" data-bind="text: TestName"></a>
            </span>
            <span data-bind="visibleAndSelect: $root.isTaskSelected($data)">
                <input data-bind="value: name, event: { blur: $root.clearTask }" />
            </span>  
        </div>
    </div>
</div>

var Task = function(first,last) {
    var self = this;
    self.firstName = ko.observable(first);
    self.lastName = ko.observable(last);
    self.TestName = ko.computed({
        read: function (){
            return self.firstName() + " " +  self.lastName();
        },
        write: function (item) {
            console.log(item);
        }
    });

    return self;
}

var ViewModel = function() {
    var self = this;
    self.testTasks = ko.observableArray([
        new Task("test","one"),
        new Task("test","two"),
        new Task("test","three")
         ]);

    self.tasks = ko.computed({
        read: function() { return self.testTasks();},
        write: function(item) {console.log(item);}
    }); 



    self.selectedTask = ko.observable();
    self.clearTask = function(data, event) {
        if (data === self.selectedTask()) {
            self.selectedTask(null);                
        }

        if (data.name() === "") {
           self.tasks.remove(data);   
        }
    };
    self.addTask = function() {
        var task = new Task("new");
        self.selectedTask(task);
        self.tasks.push(task);
    };

    self.isTaskSelected = function(task) {
       return task === self.selectedTask();  
    };
};

//control visibility, give element focus, and select the contents (in order)
ko.bindingHandlers.visibleAndSelect = {
    update: function(element, valueAccessor) {
        ko.bindingHandlers.visible.update(element, valueAccessor);
        if (valueAccessor()) {
            setTimeout(function() {
                $(element).find("input").focus().select();
            }, 0); //new tasks are not in DOM yet
        }
    }
};

ko.applyBindings(new ViewModel());

As the very author of this plugin says here, you can't use a computed observable; the sortable plugin depends on an actual observable array.

Which makes sense when you think about it: the plugin is actually manipulating the various indexes of the array as you re-sort the elements.