yelp-build misses .stack file

Gunnar Hjalmarsson <[email protected]> Wed, 12 Jun 2019 16:23:33 +0200
Newsgroups gmane.comp.gnome.documentation
Message-ID <[email protected]>
When using yelp-build to convert Mallard files to HTML, this message 
shows up:

warning: failed to load external entity "file://.../gnome-help/%2A.stack"

Wondering if someone possibly can explain what it means and/or advise on 
whether it can be ignored.

yelp-tools version: 3.32.0

-- 
Gunnar Hjalmarsson
https://launchpad.net/~gunnarhj