How do I unit test localStorage being undefined with Mocha/Sinon/Chai

I have 2 simple methods that abstract reading and writing to localStorage:

_readLocalStorage: function(key) {
    if (window.localStorage && window.localStorage.getItem(key)) {
        return JSON.parse(window.localStorage.getItem(key));
    } else {
        throw new Error('Could not read from localStorage');
    }
},

_writeLocalStorage: function(key, data) {
    try {
        window.localStorage.setItem(key, JSON.stringify(data));
    } catch (e) {
        throw new Error('Could not write to localStorage');
    }
},

Obviously, stubbing window.localStorage.getItem/setItem is simple. But what about the case where localStorage is undefined?

I've tried caching/unhinging window.localStorage (the second assertion):

describe('#_readLocalStorage', function() {
    it('should read from localStorage', function() {
        // set up
        var stub1 = sinon.stub(window.localStorage, 'getItem')
        .returns('{"foo": "bar"}');

        // run unit
        var result = service._readLocalStorage('foo');

        // verify expectations
        expect(result)
        .to.eql({foo: 'bar'});

        // tear down
        stub1.restore();
    });

    it('should throw an error if localStorage is undefined', function() {
        // set up
        var cachedLocalStorage = window.localStorage;
        window.localStorage = undefined;

        // run unit/verify expectations
        expect(service._readLocalStorage('foo'))
        .to.throw(new Error('Could not write to localStorage'));

        // tear down
        window.localStorage = cachedLocalStorage;
    });
});

This does not work however. Mocha/Chai seem not to catch the thrown error.

I've looked around a bit but can't find any way to handle this.