Bad UTF-8 content in GraphML output on plain text

[tla]

In constructing a simple test case for other purposes, I ran across a bug in the GraphML output of CollateX, which should be reproducible as follows:

Taras-iMac:MatthewEdessa tla$ cat first.txt 
The quick brown fox jumped over the lazy dogs.
Taras-iMac:MatthewEdessa tla$ cat second.txt 
the quick brown fox jumped over the lazy sleeping dog.
Taras-iMac:MatthewEdessa tla$ cat third.txt 
The quick brown fox jumped over the sleeping cat.
Taras-iMac:MatthewEdessa tla$ file *.txt
first.txt:  ASCII text
second.txt: ASCII text
third.txt:  ASCII text
Taras-iMac:MatthewEdessa tla$ ~/bin/collatex -t -f graphml first.txt second.txt third.txt > test.xml

Here is a zip file of the output - you can see an enormous blob of bad data in the middle of node 14.
test.xml.zip