    def test_walk_after_parse_failure(self):
        # This used to be an issue because libxml2 can leak empty namespaces
        # between failed parser runs.  iterwalk() failed to handle such a tree.
        try:
            etree.XML('''<anot xmlns="1">''')
        except etree.XMLSyntaxError:
            pass
        else:
            assert False, "invalid input did not fail to parse"

        et = etree.XML('''<root>  </root>''')
        try:
            ns = next(etree.iterwalk(et, events=('start-ns',)))
        except StopIteration:
            # This would be the expected result, because there was no namespace
            pass
        else:
            # This is a bug in libxml2
            assert not ns, repr(ns)
A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.