cant get upstream.current_upstream_name to work

tried different variants of configuration, no change
config:
$upstream_name used in log_format

server {
.. 
set $upstream_name 'undef';
..
.. 
}
location @siteapi_common {
 ..
  log_by_lua_block {
  ngx.var.upstream_name = "test"
  local upstream = require "ngx.upstream"
  ngx.log(ngx.INFO, "upstream = " .. tostring(upstream.current_upstream_name()))
  local upstream_name = upstream.current_upstream_name()
  ngx.var.upstream_name = upstream_name
  }
..
    proxy_pass $backend_scheme://_bk_api_common;
}

expect to see upstream name (_bk_api_common) in logs but got 'test' string and following in error.log:

2016/12/29 16:13:16 [error] 12547#12547: *20 failed to run log_by_lua*: log_by_lua(api_fallback:36):4: no srv conf for upstream
stack traceback:
        [C]: in function 'current_upstream_name'
        log_by_lua(api_fallback:36):4: in function <log_by_lua(api_fallback:36):1> while logging request, client: ip, server:test, request: "GET /_api/deals/", host: "dev-10--www.testsite"

upstreams are working fine.
What can cause 'no srv conf for upstream' error?

Start with the current_upstream_name() entry point and the supplied nginx configuration, then reproduce the log_by_lua error with the named upstream and proxy_pass setup. Trace how the module finds the upstream server configuration; done means identifying why no srv conf is found or documenting the required configuration, with the reported upstream name appearing in logs.