GetElementById returns nothing

[neotoxic]

I am loading my XML file into an XMDocument Object and I am trying to use GetElementById to locate a specific node. However, GetElementById is returning 'nothing'.

I have read somewhere that this might be to do with the fact that my ID attribute is not defined as an ID identifier. So I have appended the a document definition to my XML file, but still no luck.

My XML looks like this.

<!DOCTYPE root [ <!ELEMENT e (root*) >
 <!ATTLIST e id ID #REQUIRED>
]>
 
<root>

 <data_value id="holder_1">

      This is the value for container 1

 </data_value>
 
 <data_value id="holder_3">
 
      This is the value for container 3
 
 </data_value>

</root>

If someone could tell me how to get GetElementByID working correctly I would be very grateful.