DoH: error: attempt to call a userdata value stack traceback

Thanks for adding DoH support in your latest release. We started the process to migrate our current public DoH server to knot-resolver. Our knot-resolver test setup works so far but we see the following entries in our logs when pointing (a single) firefox (in GET mode) to the endpoint (which is behind nginx):

 [worker.background] error: attempt to call a userdata value stack traceback:
 #011[C]: in function 'wait'
 #011/usr/share/lua/5.1/cqueues/condition.lua:13: in function 'wait'
 #011/usr/lib/knot-resolver/kres_modules/http_doh.lua:102: in function 'data'
 #011/usr/lib/knot-resolver/kres_modules/http.lua:177: in function </usr/lib/knot-resolver/kres_modules/http.lua:160>
 #011[C]: in function 'yieldable_pcall'
 #011/usr/lib/knot-resolver/kres_modules/http.lua:232: in function </usr/lib/knot-resolver/kres_modules/http.lua:207>
 #011[C]: in function 'yieldable_pcall'
 #011/usr/share/lua/5.1/http/server.lua:159: in function </usr/share/lua/5.1/http/server.lua:158>

The frequency of these log events varies but we see multiple occurrences with a single browser.

firefox (network.trr.useGET = true) -> nginx -> knot-resolver

knot-resolver 4.0.0