undefined variable `$affinity` when running with simple yaml config

Full error message:
`run: adapting config using yaml: template: yaml:3: undefined variable "$affinity"`

Config which does not contain that variable:
```
apps:
layer4:
servers:
mdath:
listen:
- "127.0.0.1:443"
routes:
- match:
- tls:
sni:
- "*.host.com"
handle:
- handler: "proxy"
upstreams:
- dial:
- "localhost:4433"
```

Not sure what could be wrong given the simplicity and lack of error context but posting here in case someone has the answer!