diff --git a/gateway/conf.d/apicast.conf b/gateway/conf.d/apicast.conf
index 83d7fca4d..9c9fd5d47 100644
--- a/gateway/conf.d/apicast.conf
+++ b/gateway/conf.d/apicast.conf
@@ -1,7 +1,7 @@
 set_by_lua $user_agent 'return require("apicast.user_agent")()';
 set_by_lua_block $deployment {
   local user_agent = require('apicast.user_agent')
-  return user_agent.platform() .. '+' .. user_agent.deployment()
+  return string.format('%s+%s', user_agent.platform(), user_agent.deployment())
 }
 
 # TODO: enable in the future when we support SSL
diff --git a/gateway/src/apicast/user_agent.lua b/gateway/src/apicast/user_agent.lua
index 21decf353..bb170867b 100644
--- a/gateway/src/apicast/user_agent.lua
+++ b/gateway/src/apicast/user_agent.lua
@@ -2,6 +2,7 @@ local ffi = require 'ffi'
 local env = require 'resty.env'
 
 local setmetatable = setmetatable
+local format = string.format
 
 local _M = {
   _VERSION = require('apicast.version')
@@ -15,7 +16,7 @@ end
 -- User-Agent: Mozilla/<version> (<system-information>) <platform> (<platform-details>) <extensions>
 
 function _M.call()
-  return 'APIcast/' .. _M._VERSION .. ' (' .. _M.system_information() .. ') ' .. (_M.platform() or '')
+  return format('APIcast/%s (%s) %s', _M._VERSION, _M.system_information(), _M.platform() or 'Unknown')
 end
 
 function _M.system_information()