PURL + Path Aliases = Redirect loop

I'm trying to create a 'popup' PURL, so that I can pop things up in a window, and theme them a little differently but not have to worry about keeping track of the fact that it is in a popup window.

So I'd like to be able to popup:
/popup/node/123/edit
and for it to show the node edit form.

I've got a module that should set up the PURL properly:

/**
 * Implementation of hook_purl_provider().
 */
function MODULENAME_purl_provider() {
  $items = array();
  $items['MODULENAME'] = array(
    'name' => t('Popups'),
    'description' => t('Is the window a popup window.'),
    // TODO: This callback should set the page template:
    'callback' => 'MODULENAME_this_is_a_popup',
    //'callback arguments' => array($type),
    'example' => 'popup',
  );
  return $items;
}

/**
 * Implementation of hook_purl_modifiers().
 */
function MODULENAME_purl_modifiers() {
  $items = array();
  $items['MODULENAME'] = array(
    array('value' => 'popup', 'id' => 1),
  );
  return $items;
}

And it works when I go to any page that is not path aliased, as soon as the path has an alias (i.e. node/123 aliased to: content/my-node) I get a redirection loop. Am I missing something or is PURL fundamentally incompatible with path aliases?