Doesn't play well with Symbols (generators?)

Great library. I have a pretty esoteric case, but the failure seems to be real.

This code:

let context = {
    * [Symbol.iterator]() {
        yield Promise.resolve(1);
        yield Promise.resolve(2);
    }
};

for (let P of context) {
    console.log(P.value());
}

// 1
// 2

Fails if wrapped by Immutable:

let context = Immutable({
    * [Symbol.iterator]() {
        yield Promise.resolve(1);
        yield Promise.resolve(2);
    }
});

...

for (let P of context) {
              ^
TypeError: undefined is not a function

I'm not sure if it is the generator or the symbol usage (or computed property keys or the compact object method signature) that's causing the problem.

Running on Node (5.x), with --harmony flag. Also, bluebird is the Promise implementation.