Index: misc/autocomplete.js
===================================================================
RCS file: /cvs/drupal/drupal/misc/autocomplete.js,v
retrieving revision 1.35
diff -u -9 -p -r1.35 autocomplete.js
--- misc/autocomplete.js	29 Jan 2010 22:40:41 -0000	1.35
+++ misc/autocomplete.js	5 Mar 2010 08:45:00 -0000
@@ -109,19 +109,19 @@ Drupal.jsAC.prototype.select = function 
 };
 
 /**
  * Highlights the next suggestion.
  */
 Drupal.jsAC.prototype.selectDown = function () {
   if (this.selected && this.selected.nextSibling) {
     this.highlight(this.selected.nextSibling);
   }
-  else {
+  else if (this.popup) {
     var lis = $('li', this.popup);
     if (lis.size() > 0) {
       this.highlight(lis.get(0));
     }
   }
 };
 
 /**
  * Highlights the previous suggestion.