Bind error inside a popover bind

[win2cs]

<a href="#"  data-bind="popover: {template: 'test-template', title: 'Oh Yea'}"> 
    <span class="field-name" data-bind="text: $data.key"></span>
    <span data-bind="text: $data.value"></span>
</a>

code like this doesn't work?

any suggestions?

Thanks