Multi-friend-input not working in Facebook dialog

Question

I'm trying to get the following to work, but I've had no success:

<fb:fbml>
<fb:dialog id="friends_wallpost_dialog" cancel_button=1>
  <fb:dialog-title>Post to your friends Wall</fb:dialog-title>
  <fb:dialog-content>
    <form id="friends_wallpost_form" promptpermission="publish_stream">
      Select friends:
      <fb:multi-friend-input width="250px" border_color="#8496ba"  />
    </form>
</fb:dialog-content>
</fb:dialog>
<a href="#" clicktoshowdialog="friends_wallpost_dialog" style="border-top: solid 1px">Test</a>
</fb:fbml>

The popup dialog appears and the multi-friend-input is rendered, however, I can't type anything into the friend selector. I've had similar problems getting the multi-friend-selector to work in a request form inside a dialog box. What am I doing wrong?

Edit: I've tried with and without the fb:fbml enclosing tags. I'm using an FBML Canvas page.