Enabling TAGS_SAVE_AS causes site build to fail

I'm trying to enable the Pelican tag-based navigation for www.apache.org
(https://github.com/apache/www-site/issues/196) and as a first step I tried enabling the TAGS_SAVE_AS feature, by hardcoding it for now, setting `TAGS_SAVE_AS = 'tags.html'` in `bin/pelican.auto.ezt`

Just doing this causes a site build of the sibling `template-site` repository to fail with

WARNING Caught exception: log.py:91 'page' is undefined".

It looks like something in this module is getting in the way, but I know little about Pelican and even less about Python so I'm not sure where to look. Pointers are welcome.

Here's my failure scenario:

$ cd infrastructure-pelican
$ git diff bin/pelican.auto.ezt
...
-TAGS_SAVE_AS = ''
+TAGS_SAVE_AS = 'tags.html'
...

$ docker build -t pelican-asf2 .

# try to build https://github.com/apache/template-site
$ cd ../template-site
$ rm -rf site-generated && docker run -it -p12345:8000 -v $PWD:/site pelican-asf2
...
WARNING Caught exception:
page' is undefined".

And the `site-generated` folder is not created, nothing is generated.