json log-req-encoder does not seem to work

[muayyad-alsadi]

trafficstars

the following works

req-logger = file:/tmp/out.log
log-format = %(addr) - %(user) [%(ltime)] "%(method) %(uri) %(proto)" %(status) %(size) "%(referer)" "%(uagent)"

changing it to

req-logger = file:/tmp/out.log
log-req-encoder = json

expected result a json with all of addr, user, ...etc. actual result nothing logged