Panic when building a nix file with an unexpected type

[uhthomas]

Given hive.nix

{ pkgs, ... }: { }

Colmena will panic with

$ colmena build 
[INFO ] Enumerating nodes...
error: value is a function while a set was expected
(use '--show-trace' to show detailed location information)
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: NixFailure { exit_code: 1 }', src/command/apply.rs:122:50
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[NeverBehave]

What's the expected behavior here? e.g. a more detailed explanation over structures?

Colmena currently do support function so I think this indeed a bit confusing