XML parsing error in AJAX response from a servlet

I am using AJAX for validation. I am using servlet in server side.I am writing in the response as follows

response.setContentType("text/xml");
        response.setHeader("Cache-Control", "no-cache");
        out.write("<valid>true</valid>");

And I am getting the response as follows

 var message = xmlhttp.responseXML.getElementsByTagNam开发者_如何学Pythone("valid")[0];
            alert(message.value);

But I am getting a error:

XML parsing error: no element found

Location: moz-nullprincipal:{0ccce530-8dbe-4754-99a4-5d0f12484d56}

Line Number 1, Column 1:.

I don't know what is the problem can anyone help, please?