Troubleshooting XML error returned to client browser

Troubleshooting XML error returned to client browser

I'm new to Ajax.  I'm getting a parse error in $.ajax.  The xml file I am feeding it is dead simple, and validates just fine elsewhere.  What else can I do to see what is wrong with the xml file?

Here's my problem function:
  1. $(document).ready(function(){
  2.     $("#signup").submit(function(){
  3.     alert("post is about to be called");
  4.     $.ajax({
  5.         url: "php/signup.php",
  6.         type: "POST",
  7.         datatype: "xml",
  8.         data: {
  9.         f_email: $("#f_email").val(),
  10.         f_firstname: $("#f_firstname").val(),
  11.         f_lastname: $("#f_lastname").val(),
  12.         ...
  13.         f_other: $("#f_other").val()
  14.         },
  15.         success: function(xml) {
  16.             addMessages(xml);
  17.             },
  18.         error: function(httpreq,textstat,error_text) {
  19.             alert(httpreq);
  20.             alert(textstat);
  21.             alert(error_text);
  22.             addMessages(httpreq);
  23.             }}); 
  24.         return false;
  25.     });
  26. });

The records make it out of the post function to the php/singup.php file.  That file creates the records in the database and replies with this XML file (I have a function in signup.php that writes the xml output to a file after it sends it to the client):
  1. <?xml version="1.0"?>
  2. <response>
  3.         <message>b</message>
  4. </response>
Yes, I'm literally just sending back the letter 'b' to signify success (on an updated record.  'a' signifies a new record and 'c' signifies an error inserting the sql.)

My PHP file has this header, which works on another page that also uses the $.ajax function:
  1. header("Content-type: text/xml");
When my $.ajax receives the xml file it triggers the error function.  Those three alerts (lines 19-21) read:
[Object XMLHttpRequest]
parseerror
TypeError: Object required

How can I get more information about why $.ajax thinks that [Object XMLHttpRequest] is not an object?

Thank you!