serde deserialization error for skipped fields

Cargo.toml

[package]
name = "serde_test"
version = "0.1.0"

[dependencies]
serde = "0.9"
serde_json = "0.9"
serde_derive = "0.9"

main.rs

#[macro_use]
extern crate serde_derive;
extern crate serde_json;
extern crate serde;

use std::fs::File;
use std::io::prelude::*;

#[derive(Debug, Deserialize)]
struct User {
    name: String,
    // don't care about the "fields" field at all
    // fields: serde_json::Value,
}

fn main() {
    let json = File::open("./users.json").unwrap();
    let obj = serde_json::from_reader::<File, Vec<User>>(json);
    println!("{:?}", obj);
    // fails with Err(Syntax(Message("invalid type: map, expected a sequence"), 1, 2))
    // because "fields" goes from a Map to a Vec
}

users.json

{"users":[{"name": "bob", "fields": {"a": "b"}}, {"name": "chris", "fields": []}]}