That would be because the element in question doesn't have a namespace, and so there's no namespace URI.
Of course I have to say that without being able to see the XML in question, so I can't tell why you think it's wrong. Perhaps you could explain what you think
you should see there?
And the documentation of that method also says this:
Any or all of these may be provided, depending on the values of the http://xml.org/sax/features/namespaces and the http://xml.org/sax/features/namespace-prefixes properties
You didn't mention what the values of those properties were, either.