to2false opened a new issue #3330:
URL: https://github.com/apache/apisix/issues/3330
### Issue description
### Environment
* apisix version (cmd: `apisix version`): 2.2
* OS: (cmd: `uname -a`) Linux gateway 3.10.0-957.el7.x86_64 #1 SMP Thu Nov 8
23:39:32 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
* OpenResty / Nginx version: (cmd: `nginx -V` or `openresty -V`)
### Minimal test code / Steps to reproduce the issue
1. request GET /apisix/prometheus/metrics
2. it's not accessible
### What's the actual result? (including assertion message & call stack if
applicable)
broken
```
2021/01/19 02:10:44 [error] 43#43: *10729051 failed to run
header_filter_by_lua*: /usr/local/apisix/apisix/core/lrucache.lua:159: invalid
value (nil) at index 1 in table for 'concat'
stack traceback:
[C]: in function 'concat'
/usr/local/apisix/apisix/core/lrucache.lua:159: in function 'plugin_ctx'
/usr/local/apisix/apisix/plugins/cors.lua:168: in function 'phase_func'
/usr/local/apisix/apisix/init.lua:164: in function 'run_plugin'
/usr/local/apisix/apisix/init.lua:625: in function 'common_phase'
/usr/local/apisix/apisix/init.lua:662: in function
'http_header_filter_phase'
header_filter_by_lua:2: in main chunk, client: 10.9.6.206, server: ,
request: "GET /apisix/prometheus/metrics HTTP/1.1", host: "10.9.6.225"
```
### What's the expected result?
access ok
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