Can't access custom nixos configuration attr

[Radvendii]

Most nix commands and tools seem to have a default search path that they look at, including nixos-shell. For example
nixos-shell --flake .#foo searches at .#nixosConfigurations.foo first.

However, most nix commands and tools will fall back to searching at the top level if that fails. So if I do
nix build .#my.custom.path.foo it will first try to find packages.<system>.my.custom.path.foo, but if that fails it will look for plain my.custom.path.foo before giving up.

nixos-shell does not do this, it requires the configurations to be at specific paths.

I'm thinking this might be because nixos-shell takes two kinds of inputs: configurations and modules, and it can only tell the difference by the path. One option is to have a default (I would guess configuration). But also this could be specified at the command line with a --module or --configuration flag.