The feedreader docs include the line:

*Any field under the tag in the feed can be used for example tigger.event.data.content will get the body of the feed entry.

This is incorrect, as it causes home assistant to fail the trigger, with the error homeassistant.exceptions.TemplateError: UndefinedError: 'trigger' is undefined.
I can't seem to find what the correct version of this would be, as information on the feedreader platform is very difficult to come by. The feedreader platform itself does show up under the states tab either, making this quite difficult to figure out!

Edit: An example of the failing config can be found on my GitHub repo