XXE Injection in docx: entity not defined

My goal is to create a docx file that, when uploaded to a server and parsed there, causes the parser to fetch my url so I know it worked.

Unfortunately, I only have Libre Office and not MS Office at my hands. When I open the file with Libre Office, I get an error:

SAXException: [word/document.xml line 2]: Entity ‘xxe’ not defined

Seems like I did something wrong with my XML syntax, but I can’t figure out what.

The document.xml in the file starts like this:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?><!DOCTYPE foo [ <!ENTITY xxe SYSTEM "thisismyurl">]><w:document (...)

(Replaced my actual url for readability)

And then later, I have:

<w:t>&xxe;</w:t>(...)

I created my file with docem. Using the predefined docem payloads results in the same error. Any idea what could have gone wrong? Thank you a lot!