This was caused by an circular reference in an untested code path
in getViewData, added some months ago (lines 211-212 in 260283e2:
"lib,cli,ui,web: Introduce ReportSpec, which holds ReportOpts, the day of the report, and the parsed Query.").
This kind of bug unfortunately seems to be easy to create in Haskell,
and sometimes quite tedious to track down.
For this particular case, we should add a suitable test to the hledger-web
test suite, and/or set up a pre-release human test script, like hledger-ui's.
The bug was very obvious to the first downloader of hledger-web 1.20,
but it seems over the last three months nobody used a hledger-web built
from master. Providing and encouraging use of nightly builds could also help.
69edf9003e