XMLHttpRequest for "HEAD" dumps "no element found" errors

I use somehing like this:

var xmlhttp = new XMLHttpRequest();
xmlhttp.open("HEAD", aURL, false);
xmlhttp.onreadystatechange = function() {
  if (xmlhttp.readyState == 4) {
   return xmlhttp.getResponseHeader("Last-Modified");
  }
}
xmlhttp.send(null);

and that works, but it throws JavaScript errors like this:

Error: [JavaScript Error: "no element found" {file: 
"http://mozillazine.org/atom.xml" line: 1 column: 1 source: "
^"}]
Source File: http://mozillazine.org/atom.xml
Line: 1, Column: 1
Source Code:
^

Why is that?
Note that I looked for "no element found" and found this:

http://lxr.mozilla.org/seamonkey/search?string=no%20element%20found

but only the headers are send, not the file, so why does it spit out 
this kind of XML parser errors?

Michael