--- shorten.module
+++ shorten.module
@@ -414,7 +414,7 @@
     '#value' => '<div id="shorten_replace">',
   );
   drupal_add_js(drupal_get_path('module', 'shorten') .'/shorten.js');
-  if ($_SESSION['shorten']) {
+  if (isset($_SESSION['shorten'])) {
     $form['shortened_url'] = array(
       '#type' => 'textfield',
       '#title' => t('Shortened URL'),