diff --git a/modules/taxonomy/taxonomy.pages.inc b/modules/taxonomy/taxonomy.pages.inc
index 0cca252..fb55cd0 100644
--- a/modules/taxonomy/taxonomy.pages.inc
+++ b/modules/taxonomy/taxonomy.pages.inc
@@ -74,10 +74,21 @@ function taxonomy_term_feed($term) {
 }
 
 /**
- * Helper function for autocompletion
+ * AJAX page callback for taxonomy tag autocompletion.
+ *
+ * @param $field_name
+ *   The name of the tag field.
+ * @param $tags_typed
+ *   A comma-separated list of tags that the user has entered into the field.
  */
 function taxonomy_autocomplete($field_name, $tags_typed = '') {
-  $field = field_info_field($field_name);
+  // Make sure the field exists and is a taxonomy field.
+  if (!($field = field_info_field($field_name)) || $field['type'] !== 'taxonomy_term_reference') {
+    // Error string. The JavaScript handler will realize this is not JSON and
+    // will display it as debugging information.
+    print t('Taxonomy field @field_name not found.', array('@field_name' => $field_name));
+    exit;
+  }
 
   // The user enters a comma-separated list of tags. We only autocomplete the last tag.
   $tags_typed = drupal_explode_tags($tags_typed);
diff --git a/modules/taxonomy/taxonomy.test b/modules/taxonomy/taxonomy.test
index aa7cc2e..242fca5 100644
--- a/modules/taxonomy/taxonomy.test
+++ b/modules/taxonomy/taxonomy.test
@@ -638,6 +638,15 @@ class TaxonomyTermTestCase extends TaxonomyWebTestCase {
     $input = substr($term3->name, 0, 3);
     $this->drupalGet('taxonomy/autocomplete/taxonomy_' . $this->vocabulary->machine_name . '/' . $input);
     $this->assertRaw('{"' . $term3->name . '":"' . $term3->name . '"}', t('Autocomplete returns term %term_name after typing the first 3 letters.', array('%term_name' => $term3->name)));
+
+
+    // Test taxonomy autocomplete with a nonexistent field.
+    $field_name = $this->randomName();
+    $tag = $this->randomName();
+    $message = t("Taxonomy field @field_name not found.", array('@field_name' => $field_name));
+    $this->assertFalse(field_info_field($field_name), t('Field %field_name does not exist.', array('%field_name' => $field_name)));
+    $this->drupalGet('taxonomy/autocomplete/' . $field_name . '/' . $tag);
+    $this->assertRaw($message, t('Autocomplete returns correct error message when the taxonomy field does not exist.'));
   }
 
   /**