Fields inside of fieldset code are being filtered out automatically.

I have some fields in my form set up something like this:

  $form['details'] = array(
    '#type' => 'fieldset',
    '#title' => t('Details'),
    '#tree' => FALSE,
  );  

  $form['details']['left'] = array(
    '#prefix' => '<div class="left">',
    '#suffix' => '</div>',
    '#tree' => FALSE,
  );
  $form['details']['left']['duration'] = array(
    '#type' => 'textfield',
    '#title' => "Time to Enter",
    '#default_value' => $node->duration,
    '#required' => TRUE,
  );

Any elements set up inside of $form['details']['left'] do not show up but are filtered out automatically.

If I edit the field, for example by changing the 'duration' field to as follows:

  $form['duration'] = array(
    '#type' => 'textfield',
    '#title' => "Time to Enter",
    '#default_value' => $node->duration,
    '#required' => TRUE,
  );

Then the field 'duration' will appear on the form as intended.

I think the conflict occurs inside of the function formfilter_filter_form around line 184.

These lines:

    foreach (element_children($form) as $key) {
      if (!in_array($key, $filtered)) {
        _formfilter_filter_element($form[$key]);
      }
    }

appear to loop through anything in $form that doesn't start with a '#'. So, it will run through 'details', but it seems to skip the items nested inside of 'details' such as, in this example, 'duration'.

What should I do to allow the formfilter module to include fields which are nested such as $form['details']['left']['duration'] ?