Incomplete XInclude Resolution for 2nd Level?

check.py

#!/usr/bin/env python3

import sys
import textwrap
from lxml import etree

def parse(xmlfile, xinclude=True):
  try:
    xmlparser = etree.XMLParser(collect_ids=False, recover=False)
    tree = etree.parse(xmlfile, parser=xmlparser)
    if xinclude:
          tree.xinclude()
    return 0
  except (etree.XMLSyntaxError, etree.XIncludeError) as err:
    print("ERROR: %s" % err, file=sys.stderr)
    print(textwrap.indent(str(err.error_log), prefix="       "), file=sys.stderr)
    return 10

if __name__ == "__main__":
  result = parse(sys.argv[1])
  sys.exit(result)

first.xml

<section version="5.1"
 xmlns="http://docbook.org/ns/docbook"
 xmlns:xi="http://www.w3.org/2001/XInclude">
 <title>First Level</title>
 <xi:include href="second.xml"/>
</section>

main.xml

<article version="5.1"
 xmlns="http://docbook.org/ns/docbook"
 xmlns:xi="http://www.w3.org/2001/XInclude">
  <title>Test for nested XIncludes</title>
 <xi:include href="first.xml"/>
</article>

second.xml

<section version="5.1"
 xmlns="http://docbook.org/ns/docbook">
    <title>Second</title>
    <para>The quick brown fox.<!--</para>-->
</section>