Luci-app-fwknopd Bug

there seems to be a problem with luci-app-fwknopd. After installing you can see this message in fwknopd Luci control

/usr/lib/lua/luci/dispatcher.lua:460: Failed to execute cbi dispatcher target for entry ‘/admin/services/fwknopd’.
The called action terminated with an exception:
/usr/lib/lua/luci/template.lua:97: Failed to execute template ‘cbi/map’.
A runtime error occured: /usr/lib/lua/luci/template.lua:97: Failed to execute template ‘cbi/tsection’.
A runtime error occured: /usr/lib/lua/luci/template.lua:97: Failed to execute template ‘cbi/ucisection’.
A runtime error occured: [string “/usr/lib/lua/luci/view/cbi/ucisection.htm”]:15: attempt to call field ‘json_encode’ (a nil value)
stack traceback:
[C]: in function ‘assert’
/usr/lib/lua/luci/dispatcher.lua:460: in function ‘dispatch’
/usr/lib/lua/luci/dispatcher.lua:141: in function

OpenWrt omnia 15.05 r47055 / LuCI 526a8767846acbe57c521912b35feb4d97354db6 branch (git-18.145.30016-526a876)

Having installed fwknopd and luci-app-fwknopd, I find that the luci Services/Firewall Knock Daemon shows an error on-screen, see attached screenshot.

problem

This seems the same as previously reported in Sep 2017 at Luci-app-fwknopd Bug