Unexpected brackets in to_value of a newtype struct

[dtolnay]

#![feature(custom_derive, plugin)]
#![plugin(serde_macros)]

use std::collections::BTreeMap;

extern crate serde_json;

#[derive(Serialize, Deserialize)]
struct Bar(BTreeMap<String, i32>);

fn main() {
    let mut b = Bar(BTreeMap::new());
    b.0.insert(String::from("foo"), 123);

    // prints {"foo":123}
    println!("{}", serde_json::to_string(&b).unwrap());

    let mut outer = BTreeMap::new();
    outer.insert(String::from("outer"), serde_json::to_value(&b));

    // prints {"outer":[{"foo":123}]}
    println!("{}", serde_json::to_string(&outer).unwrap());
}

I would expect the second print to be {"outer":{"foo":123}}.