Index: modules/taxonomy/taxonomy.pages.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/taxonomy/taxonomy.pages.inc,v
retrieving revision 1.51
diff -u -p -r1.51 taxonomy.pages.inc
--- modules/taxonomy/taxonomy.pages.inc	10 Feb 2010 06:28:10 -0000	1.51
+++ modules/taxonomy/taxonomy.pages.inc	1 Jun 2010 11:40:46 -0000
@@ -74,7 +74,9 @@ function taxonomy_term_feed($term) {
  * Helper function for autocompletion
  */
 function taxonomy_autocomplete($field_name, $tags_typed = '') {
-  $field = field_info_field($field_name);
+  if (!$field = field_info_field($field_name)) {
+    return drupal_json_output(array());
+  }
 
   // The user enters a comma-separated list of tags. We only autocomplete the last tag.
   $tags_typed = drupal_explode_tags($tags_typed);