Tags not working in CompositeIndexer

[peppergrower]

I have two models, Submission and Response. I have defined an indexer
for each of them in index.py, as demonstrated in the tutorial. (Great
tutorial, by the way: the documentation is pretty clear. And thank
you very much for the time you have spent on djapian!)

I also defined complete_indexer as shown in the tutorial, namely:

import djapian
complete_indexer = djapian.CompositeIndexer(Submission.indexer,
Response.indexer)

...after defining the other indexers and calling
djapian.space.add_index().

Now, for my question: when I query complete_indexer with a simple text
query (e.g., complete_indexer.search('testing').prefetch() ), I get
the results I expect. However, when I query complete_indexer with a
query that includes a tag, I get zero results: even if running the
query (with the tag) directly on Submission.indexer and
Response.indexer does return results. The same tag is even defined in
both Submission.indexer and Response.indexer.

Why is this? Shouldn't complete_indexer return the combination of
results from Submission.indexer and Response.indexer? Do tags not
work with CompositeIndexer, or is there a bug?