Undefined index: target in include() while Drupal caches are turned on

Hello,

I'm working on an image slideshow. It's based on a field collaction that might have an image, an image title and a link. Everything works fine so far. But everytime I activate the Drupal caches I receive the following error message:

Notice: Undefined index: target in include() (on line 38 in /.../node--slideshow.tpl.php).

Here is line 38:
echo '<div class="slideshow-element">'. l(theme('image', array('path' => image_style_url('slideshow', $element->field_bilder['und'][0]['uri']), 'alt' => $element->field_bilder['und'][0]['title'], 'title' => $element->field_bilder['und'][0]['title'])). $image_title, $element->field_link['und'][0]['url'], array('html' => true, 'attributes' => array('target' => $element->field_link['und'][0]['attributes']['target']))) .'</div>';

And here the complete code:

  if (!empty($content['field_slideshow_element']['#items'][0]['value'])) {
    echo '<div class="slideshow-wrapper"><div class="slideshow">';
    foreach ($content['field_slideshow_element']['#items'] as $slideshowelement) {
      $collection = entity_load('field_collection_item', $slideshowelement);
      foreach ($collection as $element) {
        $image_title = '';
        if (!empty($element->field_bilder['und'][0]['title'])) {
          $image_title = '<span class="slideshow-text">'. $element->field_bilder['und'][0]['title'] .'</span>';
        }
        if (!empty($element->field_bilder['und'][0]['fid']) && !empty($element->field_link['und'][0]['url'])) {
          // HERE COMES LINE 38
          echo '<div class="slideshow-element">'. l(theme('image', array('path' => image_style_url('slideshow', $element->field_bilder['und'][0]['uri']), 'alt' => $element->field_bilder['und'][0]['title'], 'title' => $element->field_bilder['und'][0]['title'])). $image_title, $element->field_link['und'][0]['url'], array('html' => true, 'attributes' => array('target' => $element->field_link['und'][0]['attributes']['target']))) .'</div>';
        }
        elseif (!empty($element->field_bilder['und'][0]['fid']) && empty($element->field_link['und'][0]['url'])) {
          echo '<div class="slideshow-element">'. theme('image', array('path' => image_style_url('slideshow', $element->field_bilder['und'][0]['uri']), 'alt' => $element->field_bilder['und'][0]['title'], 'title' => $element->field_bilder['und'][0]['title'])). $image_title .'</div>';
        }
      }
    }
    echo '</div><div class="slideshow-prev"></div><div class="slideshow-next"></div></div>';
  }

Do I have a mistake in my code or is it a bug? It's suspect because I only receive this error when I turn the caches on. Maybe I'm overseeing something very trivial.

Thank you!
Stefan