can.compute(initialValue, setter) doesn't work in can.Component

I'm trying to use can.compute seter with initial value.
Initial value is works correctly, but setter doesn't works.

I mean this construction "can.compute(initialValue, setterFn)" behaves like this "can.compute(initialValue)"

Check yourself
http://jsbin.com/OjEhIsaG/watch?js,output

1. can.Component.extend({
2.     tag: "my-tag",
3.     template: "<button>click</button> {{val}}",
4.     scope : {
5.         val : can.compute('default val', function(newVal) {
6.             alert('this function will never be called');
7.             return 'I am the real value';
8.         })
9.     },
10.     events : {
11.         'button click' : function(el) {
12.             this.scope.val(Math.random());
13.         }
14.     }
15. });
16. var template = can.view.mustache("<my-tag></my-tag>");
17. $("body").html(template());