[SOLVED] Problem Using DOM after changing header information

[Somme]

I have a function in javascript to make writing the code for an image a little quicker:

 

var currentcontent;
var imgalt;
var imgtitle;
var theimg;
var imgurl

function img()
{
currentcontent = document.newpost.entrycontent.value
imgurl = prompt("Enter the image url:");
imgalt = prompt("Enter alternative text");
imgtitle = prompt("Enter image title");
theimg = "<img src=\"" + imgurl + "\" alt=\"" + imgalt + "\" title=\"" + imgtitle +"\" />";
document.newpost.entrycontent.value = currentcontent + theimg;  
}

 

This works fine apart from when I change the header from:

header('Content-type:text/html; charset=utf-8'); to

header('Content-type: application/xhtml+xml; charset=utf-8');

It simply doesn't work. In my error console it simply says 'document.newpost has no properties'

 

Any help could be much appreciated =)