Error deserializing tagged enum containing map with integer keys

[manforowicz]

I get a runtime error when trying to deserialize a tagged enum that contains a map with integer keys. Here's a reproducible example:

use serde::{Deserialize, Serialize};
use std::collections::BTreeMap;

#[derive(Serialize, Deserialize)]
#[serde(tag = "type")]
enum Msg {
    Map { map: BTreeMap<i32, i32> },
}

fn main() {
    let msg = Msg::Map {
        map: [(1, 2), (3, 4)].into(),
    };

    let serialized = serde_json::to_string_pretty(&msg).unwrap();
    println!("Serialized:\n{serialized}");

    let _received: Msg = serde_json::from_str(&serialized).unwrap();
}

It panics with:

Error("invalid type: string \"1\", expected i32", line: 0, column: 0)

Playground link.

However, I don't get an error when I remove the #[serde(tag = "type")], or when I use non-integer keys. Is this expected behavior?