How to output X-Forwarded-For by Logging policy?

Issue

• How to output X-Forwarded-For by Logging policy in Red Hat 3scale API Management?

• The User-Agent header can be output by this syntax: {{req.headers['User-Agent']}}. But when setting {{req.headers['X-Forwarded-For']}} APICast throws this exception:

  2020/08/25 03:29:45 [error] 20#20: *617 failed to run log_by_lua*: /usr/local/share/lua/5.1/liquid.lua:1922: attempt to index field 'nodetab' (a nil value)
  stack traceback:
  /usr/local/share/lua/5.1/liquid.lua:1922: in function 'raise_error'
  /usr/local/share/lua/5.1/liquid.lua:2045: in function 'visit'
  /usr/local/share/lua/5.1/liquid.lua:2018: in function 'render'
  /opt/app-root/src/src/apicast/policy/logging/logging.lua:102: in function 'get_log_line'
  /opt/app-root/src/src/apicast/policy/logging/logging.lua:154: in function </opt/app-root/src/src/apicast/policy/logging/logging.lua:137>
  /opt/app-root/src/src/apicast/policy_chain.lua:200: in function </opt/app-root/src/src/apicast/policy_chain.lua:197>
  /opt/app-root/src/src/apicast/policy_chain.lua:200: in function 'log'
  log_by_lua(lua_FUgRvX:471):3: in main chunk while logging request, client: 10.116.0.1, server: _, request: "GET /?user_key=xxxx HTTP/1.1", upstream: "https://api.example.com:443/?user_key=xxxx", host: "api.example.com"