Index: modules/user/user.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.module,v
retrieving revision 1.739
diff -u -p -r1.739 user.module
--- modules/user/user.module	7 Jan 2007 08:20:31 -0000	1.739
+++ modules/user/user.module	9 Jan 2007 23:31:09 -0000
@@ -2442,9 +2442,11 @@ function _user_forms(&$edit, $account, $
  */
 function user_autocomplete($string = '') {
   $matches = array();
-  $result = db_query_range("SELECT name FROM {users} WHERE LOWER(name) LIKE LOWER('%s%%')", $string, 0, 10);
-  while ($user = db_fetch_object($result)) {
-    $matches[$user->name] = check_plain($user->name);
+  if ($string) {
+    $result = db_query_range("SELECT name FROM {users} WHERE LOWER(name) LIKE LOWER('%s%%')", $string, 0, 10);
+    while ($user = db_fetch_object($result)) {
+      $matches[$user->name] = check_plain($user->name);
+   }
   }
   print drupal_to_js($matches);
   exit();