diff --git a/modules/taxonomy/taxonomy.pages.inc b/modules/taxonomy/taxonomy.pages.inc
index 299c7bb..975ff12 100644
--- a/modules/taxonomy/taxonomy.pages.inc
+++ b/modules/taxonomy/taxonomy.pages.inc
@@ -118,7 +118,7 @@ function taxonomy_term_feed($term) {
  * @see taxonomy_menu()
  * @see taxonomy_field_widget_info()
  */
-function taxonomy_autocomplete($field_name, $tags_typed = '') {
+function taxonomy_autocomplete($field_name = '', $tags_typed = '') {
   // If the request has a '/' in the search text, then the menu system will have
   // split it into multiple arguments, recover the intended $tags_typed.
   $args = func_get_args();
@@ -138,7 +138,7 @@ function taxonomy_autocomplete($field_name, $tags_typed = '') {
   $tags_typed = drupal_explode_tags($tags_typed);
   $tag_last = drupal_strtolower(array_pop($tags_typed));
 
-  $matches = array();
+  $term_matches = array();
   if ($tag_last != '') {
 
     // Part of the criteria for the query come from the field's own settings.
@@ -167,7 +167,6 @@ function taxonomy_autocomplete($field_name, $tags_typed = '') {
 
     $prefix = count($tags_typed) ? drupal_implode_tags($tags_typed) . ', ' : '';
 
-    $term_matches = array();
     foreach ($tags_return as $tid => $name) {
       $n = $name;
       // Term names containing commas or quotes must be wrapped in quotes.
diff --git a/modules/taxonomy/taxonomy.test b/modules/taxonomy/taxonomy.test
index 123bdce..270ed58 100644
--- a/modules/taxonomy/taxonomy.test
+++ b/modules/taxonomy/taxonomy.test
@@ -722,6 +722,13 @@ class TaxonomyTermTestCase extends TaxonomyWebTestCase {
     $this->assertFalse(field_info_field($field_name), format_string('Field %field_name does not exist.', array('%field_name' => $field_name)));
     $this->drupalGet('taxonomy/autocomplete/' . $field_name . '/' . $tag);
     $this->assertRaw($message, 'Autocomplete returns correct error message when the taxonomy field does not exist.');
+
+    // Test the autocomplete path without passing a field_name and terms.
+    // This should not trigger a PHP notice.
+    $field_name = '';
+    $message = t("Taxonomy field @field_name not found.", array('@field_name' => $field_name));
+    $this->drupalGet('taxonomy/autocomplete');
+    $this->assertRaw($message, 'Autocomplete returns correct error message when no taxonomy field is given.');
   }
 
   /**