Unexpected Behavior with Variable Interpolation in TOML Comments

[mre]

Given the following TOML config:

# key = "${FOO}"

and this main.rs:

use twelf::{config, Layer};

#[config]
struct Conf {}

fn main() {
    Conf::with_layers(&[Layer::Toml("example.toml".into())]).unwrap();
}

When I run the program, I get an error:

thread 'main' panicked at src/main.rs:7:62:
called `Result::unwrap()` on an `Err` value: ShellExpand(LookupError { var_name: "FOO", cause: NotPresent })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

It appears that variables within comments are interpolated/expanded.
This is an issue, because I would like to provide a sample config with commented settings which contain environment variables.