Using checkedValue binding with radio buttons

I'm trying to make use of the checkedValue binding introduced in knockout version 3, with radio buttons , but am not getting the behavior I expect.

Here's an example: (the viewModel has two properties; list is an array; checkedVal is an observable)

<div data-bind="foreach:list">
    <input type="radio" data-bind="
        checkedValue: {
            data: $data,
            index: $index(),
        },
        checked: $parent.checkedVal
    "/>
    <span data-bind="text: $data"></span>
</div>

JSFiddle

I expect the radio buttons to behave normally, and checkedVal to be an object containing the data and index. checkedVal is as I expect, but the radio buttons don't select. Oddly, in my actual code the behavior is inconsistent; sometimes the radio butttons work and sometimes they don't, but it consistently doesn't work in the fiddle, as far as I can see.

Is this a bug, or am I misunderstanding how this should be working?