cannot borrow `self.hash` as mutable because it is also borrowed as immutable

I'm trying to make this work

use std::collections::HashMap;

struct Test1 {
    total: u32,
    hash: HashMap<u32, u32>,
}

impl Test1 {
    fn new() -> Test1 {
        Test1 {
            total: 0,
            hash: HashMap::new(),
        }
    }

    fn add(&mut self) -> u32 {
        self.total += 1;
        self.total
    }

    fn get_or_create(&mut self, id: u32) -> u32 {
        match self.hash.get(&id) {
            Some(value) => *value,
            None => {
                let value = self.add();
                self.hash.insert(id, value);
                value
            }
        }
    }
}

fn main() {
    let mut test = Test1::new();
    println!("{:?}", test.get_or_create(1));
    println!("{:?}", test.get_or_create(1));
}

(playpen)[http://is.gd/hDLEaL]

but I get

<anon>:25:33: 25:37 error: cannot borrow `*self` as mutable because `self.hash` is also borrowed as immutable [E0502]

removing pattern matching doesn't address the problem, but I don't understand why.