variable "target" not found for writing

[Err0rZero]

variable "target" not found for writing

请问这个错误什么问题？

2018/07/09 12:32:31 [error] 28133#0: *1 lua entry thread aborted: runtime error: /usr/local/openresty/nginx/conf/x-waf/waf.lua:255: variable "target" not found for writing; maybe it is a built-in variable that is not changeable or you forgot to use "set $target '';" in the config file to define it first
stack traceback:
coroutine 0:
[C]: in function '__newindex'
/usr/local/openresty/nginx/conf/x-waf/waf.lua:255: in function 'start_jingshuishuiyue'
access_by_lua(www.test.com.conf:19):1: in function <access_by_lua(www.test.com.conf:19):1>, client: 210.14.78.174, server:www.test.com
, request: "GET / HTTP/1.1", host: "www.test.com"