[Bug] APISix 3.13.0 - balancer.lua:328: set_current_peer expects string but gets table

[khrism]

Current Behavior

APISix 3.13.0 fails to process upstream nodes configuration with error: bad argument #3 to 'set_current_peer' (string expected, got table) at balancer.lua:328.

Additional Info:

Echo plugin works perfectly (no upstream needed)

Admin API fully functional

etcd integration working

Issue occurs with both hostname and IP upstream nodes

Service-based approach also fails with same error

Expected Behavior

No response

Error Logs

2025/08/23 05:24:25 [error] 16117#16117: 11337 failed to run balancer_by_lua: /usr/local/openresty/lualib/ngx/balancer.lua:159: bad argument #3 to 'set_current_peer' (string expected, got table)
stack traceback:
[C]: in function 'error'
/usr/local/openresty/lualib/ngx/balancer.lua:159: in function 'set_current_peer'
/home/khris/apisix/balancer.lua:328: in function 'set_current_peer'
/home/khris/apisix/balancer.lua:386: in function 'run'
/home/khris/apisix/init.lua:983: in function 'http_balancer_phase'

Steps to Reproduce

1. Create route with upstream:

curl -X PUT http://127.0.0.1:9180/apisix/admin/routes/1 \
  -H "X-API-KEY: YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "uri": "/test",
    "upstream": {
        "type": "roundrobin",
        "nodes": {
            "httpbin.org:80": 1
        }
    }
}'
Access route: curl http://127.0.0.1:9080/test

Results in 500 error

### Environment

- APISix: 3.13.0
- OS: Ubuntu 22.04
- OpenResty: [output from nginx -v]
- etcd: 3.5.0
- LuaJIT: [output from luajit -v]

[juzhiyuan]

Hi @khrism, from your configuration, there are 2 issues:

1. Your target URI doesn't exist on Upstream. This means that when you send a request via http://127.0.0.1:9080/test, the gateway forwards the request to http://httpbin.org/test. However, this doesn't exist on httpbin.org, so it returns a 404 error.
2. Your configuration is missing the parameter "upstream.pass_host: node." After adding this parameter, it works, even though it returns a 404 error.

curl -X PUT http://127.0.0.1:9180/apisix/admin/routes/1 \
  -H "Content-Type: application/json" \
  -d '{
    "uri": "/test",
    "upstream": {
        "type": "roundrobin",
        "pass_host": "node",
        "nodes": {
            "httpbin.org:80": 1
        }
    }
}'

https://docs.api7.ai/apisix/reference/admin-api#tag/Upstream/paths/~1apisix~1admin~1upstreams/post

3. I think this is NOT an issue with APISIX itself. When you use cURL to access httpbin.org directly without a Host header or with a Host header that does not match httpbin.org, httpbin.org returns an error.

$ curl httpbin.org/test -H "Host: httpbin.org" -I
HTTP/1.1 404 NOT FOUND
Date: Sat, 23 Aug 2025 13:43:27 GMT
Content-Type: text/html
Content-Length: 233
Connection: keep-alive
Server: gunicorn/19.9.0
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true

$ curl httpbin.org/test -H "Host: WRONG-HOST" -I 
curl: (52) Empty reply from server