Xpath search not possible with empty prefix?

I have a GPX file in which the prefix for most elements is simply the empty string, i.e. ''.
To search for elements I use the findall function and pass the appropriate namespace.
However, I cannot search for elements with an empty prefix as demonstrated below.

import xml.etree.ElementTree as ET

xml = '''<?xml version="1.0" encoding="UTF-8"?>
<foo xmlns="some-url" xmlns:ns1="other-url">
tree = ET.fromstring(xml)
ns = {'': 'some-url', 'ns1': 'other-url'}

To search for elements I use

tree.findall('.//ns1:bar', ns)

which works correctly and gives [<Element '{other-url}bar' at 0x12128f230>]. But

tree.findall('.//baz', ns)

only returns [].

Am I doing something wrong? There is a hint in the documentation that says “Pass '' as prefix to move all unprefixed tag names in the expression into the given namespace.” But I don’t know what that is supposed to tell me. Using tree.findall('.//'':baz', ns) just throws an error.

PS.: I cannot really imagine this being a bug. But if it is one, I am happy to try to fix it and send a PR.