Problem with child elements

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Use a list item as target-element (the element for the onmouseover event)
2. Place more child DIVs in the list item
3. By moving the mouse pointer in one of the child DIVs, tooltip will appear. 
Now move the mouse pointer in another child DIV, tooltip will disappear. 
Leaving the parent element (list item) an there comes up an error like:
(translated from german)
SCRIPT5007: For the property "removeChild" no value can be retrieved: the 
object is null or undefined.

What version of the product are you using? On what operating system?
Latest version, Windows 7, IE9

Please provide any additional information below.

Example:

<ul>
  <li class="tooltip">
    <div>1</div>
    <div>2</div>
    <div>3</div>
  </li>
 ...
</ul>
<script>
$$("li.tooltip").each(
  function(element) {
    new Tooltip(element, {...});
  }
}
</script>

Original issue reported on code.google.com by Sebastia...@gmail.com on 13 Oct 2011 at 2:22