dub: Could not resolve configuration for package demo

[mw]

Very simple thing from https://dub.pm/getting-started/first-steps/

After I add dependencies, it cannot build:

```
$ cat dub.json
{
         "authors": [
                 "mw"
         ],
         "copyright": "Copyright © 2024, mw",
         "description": "msgpack-rpc-d demo.",
         "license": "MIT",
         "name": "demo",
         "dependencies": {
                 "msgpack-rpc": "~>0.1.3"
         },
         "subConfigurations": {
                 "msgpack-rpc": "default"
         }
}

$ dub build
Could not resolve configuration for package demo
```

So what I'm missing?

Thanks.