How does EvaluateXPath processor in NIFI use XPaths in its configuration?

[#1]

I have found that valid XPath expressions apparently produce empty strings in EvaluateXPath processor. In the attached screenshots only //@categoryId and //@UniqueType seem to work. I am using nifi 1.7.1 and jdk1.8.0_181.jdk. Any help would be appreciated!

@dmueller1607

@mburgess

@alopresto

[#2]

The source of the XML files is a GetFile Processor or a GenrateFlowFile Processor.

I have also tested it with an InvokeHTTP processor where it works fine. However this does not solve it for me as I am dealing with physical XML files here. However this little detail might be interesting for the error finding process.