diff -u b/menu_attributes.install b/menu_attributes.install
--- b/menu_attributes.install
+++ b/menu_attributes.install
@@ -28,7 +28,7 @@
     variable_del("menu_attributes_{$attribute}_enable");
     variable_del("menu_attributes_{$attribute}_default");
   }
-  variable_del("menu_attributes_menus");
+  variable_del('menu_attributes_menus');
 }
 
 /**
diff -u b/menu_attributes.module b/menu_attributes.module
--- b/menu_attributes.module
+++ b/menu_attributes.module
@@ -136,10 +136,10 @@
  */
 function menu_attributes_form_menu_edit_menu_alter(&$form, $form_state) {
   $menu_name = $form['menu_name']['#default_value'];
-  $attrs = variable_get('menu_attributes_menus', array());
+  $attributes = variable_get('menu_attributes_menus', array());
 
   // Set default attributes.
-  $default_attrs = !empty($attrs[$menu_name]) ? $attrs[$menu_name] : array('id' => '', 'class' => '');
+  $default_attributes = !empty($attributes[$menu_name]) ? $attributes[$menu_name] : array('id' => '', 'class' => '');
 
   $form['attributes'] = array(
     '#type' => 'fieldset',
@@ -150,13 +150,13 @@
   );
   $form['attributes']['id'] = array(
     '#type' => 'textfield',
-    '#title' => 'ID',
-    '#default_value' => $default_attrs['id'],
+    '#title' => t('ID'),
+    '#default_value' => $default_attributes['id'],
   );
   $form['attributes']['class'] = array(
     '#type' => 'textfield',
-    '#title' => 'Classes',
-    '#default_value' => $default_attrs['class'],
+    '#title' => t('Classes'),
+    '#default_value' => $default_attributes['class'],
   );
 
   $form['#submit'][] = '_menu_attributes_menu_form_submit';
@@ -173,14 +173,14 @@
   // Adds 'menu-' prefix if menu has been created.
   $menu_name = !empty($values['old_name']) ? $values['menu_name'] : 'menu-' . $values['menu_name'];
 
-  $attrs = variable_get('menu_attributes_menus', array());
+  $attributes = variable_get('menu_attributes_menus', array());
 
   // Store attributes in variable.
-  $attrs[$menu_name] = array(
+  $attributes[$menu_name] = array(
     'id' => $values['attributes']['id'],
-    'class' => $values['attributes']['class']
+    'class' => $values['attributes']['class'],
   );
-  variable_set('menu_attributes_menus', $attrs);
+  variable_set('menu_attributes_menus', $attributes);
 }
 
 /**
@@ -415,24 +415,20 @@
 }
 
 /**
- * Pre-preprocess function for template_preprocess_menu_tree().
- *
- * Adds menu name to $vars. Gets menu name from first link item.
+ * Prepares variables for theme_menu_tree().
  */
-function menu_attributes_prepreprocess_menu_tree(&$vars) {
-  $first_element_key = current(element_children($vars['tree']));
-  $first_element = $vars['tree'][$first_element_key];
-  $vars['menu_name'] = $first_element['#original_link']['menu_name'];
+function menu_attributes_prepreprocess_menu_tree(&$variables) {
+  $first_element_key = current(element_children($variables['tree']));
+  $first_element = $variables['tree'][$first_element_key];
+  $variables['menu_name'] = $first_element['#original_link']['menu_name'];
 }
 
 /**
- * Override theme_menu_tree().
- *
- * Output menu attributes.
+ * Implements theme_menu_tree().
  */
 function menu_attributes_menu_tree($variables) {
   $menus = variable_get('menu_attributes_menus', array());
-  $attributes = !empty($menus[$variables['menu_name']]) ? $menus[$variables['menu_name']] : array();
+  $attributes = !empty($menus[$variables['menu_name']]) ? array_filter($menus[$variables['menu_name']]) : array();
 
   return '<ul' . drupal_attributes($attributes) . '>' . $variables['tree'] . '</ul>';
 }