ngx.location.capture + proxy_pass return value is empty

When I try to use ngx.location.capture + proxy_pass to make a subrequest,
I find that no matter how I configure it, the return value is empty.
How can I configure it to achieve the correct return of the subrequest?

ps: I have confirmed that the sub-request has been successful, and the server returns 200

nginx version: nginx/1.14.0

lua-nginx-module-0.10.13

nginx config

...
    upstream authserver {
        server 192.168.31.152:5001;
    }
...

location /lua {
            # default_type 'text/plain';
            # content_by_lua 'ngx.say("/lua")';
            content_by_lua_file /lua/content.lua;
        }
        location /api/post/ {
            # internal;
            proxy_pass http://authserver/api/access/auth?source=;
            # health_check;
            # content_by_lua 'ngx.say("/api/post")';
            # proxy_read_timeout 3600;
        }

content.lua

local res = ngx.location.capture("/api/post/",{method=ngx.HTTP_POST,always_forward_body=true,body='',args= { arg='auth'}})
        ngx.say("request successed")
        -- ngx.say(res)
        if not res then
            for key, value in pairs(res) do
                ngx.say(key+":"+value)
            end
             -- ngx.say(res);
            ngx.say(res.state)
            ngx.say(res.body)
            -- ngx.say(res)
        else
            ngx.say("result is null")
        end

result

request successed
result is null

Start with the nginx locations and upstream configuration in the issue, then inspect content.lua and the ngx.location.capture entry point. Reproduce the request with the stated nginx and lua-nginx-module versions; done means the captured subrequest result is returned with a documented, reproducible configuration.