Cannot read env variables when field names have `_` in them

Getting the following error when trying to get an environmental variable from config-rs:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: configuration property "DEV_SERVER_ADDRESS" not found', src/app/config.rs:14:27 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

use config::Config;
use lazy_static;

lazy_static! {
    #[derive(Debug)]
    pub static ref CONFIG: Config = Config::builder()
        .add_source(config::Environment::with_prefix("APP_NAME").separator("_"))
        .add_source(config::File::with_name("AppName"))
        .build()
        .unwrap();
}

pub fn get<'a, T: serde::Deserialize<'a>>(key: &str) -> T {
    CONFIG.get::<T>(&key).unwrap()
}

ENV variable:

export APP_NAME_DEV_SERVER_ADDRESS="testtt dev addr"

Trying to get the variable in main:

#[actix_web::main]
async fn main() -> result::Result<(), io::Error> {

    let dev_server_address: String = app::config::get("DEV_SERVER_ADDRESS");
    println!("DEV_SERVER_ADDRESS: {}", dev_server_address);
...

What am I doing wrong?

Start with the reproduction in src/app/config.rs:14, especially Environment::with_prefix("APP_NAME").separator("_") and the get("DEV_SERVER_ADDRESS") entry point. Run it with APP_NAME_DEV_SERVER_ADDRESS set, then determine whether the underscore handling matches the intended environment-source behavior; done means the reported lookup either works or its expected limitation is clearly established.