Aurelia unit test new class instance

I'm trying to write a unit test for a class in my Aurelia app, as follows:

describe("MyClass", () => {
    let sut;

    beforeEach(() => {
        sut = new MyClass(new S1(), new S2());
    });

    it(('should have default value') => {
        expect(sut.getValue()).toBe('default');
        expect(sut.getValue()).not.toBe('something else');
    });

    it('should set new value', () => {
        let val = 'stack';
        sut.setValue(val);
        expect(let.getValue()).toBe(stack);
        expect(let.getValue()).not.toBe('default');
    });

    it('should not set invalid value', () => {
        let invalidVal = 'invalid value';
        sut.setValue(invalidVal);

        expect(sut.getValue()).toBe('default');
        expect(sut.getValue()).not.toBe(invalidVal);
    });
});

The test fails at 'should not set invalid value' - where expected 'stack' to be 'default'.