Adding Freefall

I've tried to add the feed for the webcomic Freefall [1] as a gwene
group, but gwene.org says

    "Not a valid feed."

(BTW, Feedbase says "Invalid group", I'm not sure if it's something else
failing (maybe I need to check the documentation about acceptable group
names there?) or if Feedbase is also saying that this feed isn't valid.)

Is this something that can be somehow worked around or is it a problem
with the feed itself?

[1] https://crosstimecafe.com/Webcomics/Feeds/Freefall.xml

-- 
Nuno Silva