Spy's thisValue not pointing to instance when spying constructor

This snippet works fine with Sinon 2.0.0 but fails with Sinon 4.2.2:

it('returns correct thisValue', function() {
    window.SomeClass = function() {
        this.counter = 0;
        this.inc = function() {
            this.counter++;
        };
        this.getCounter = function() {
            return this.counter;
        };
    };

    var spy = sinon.spy(window, 'SomeClass');
    var some = new window.SomeClass();
    some.inc();

    expect(spy.thisValues[0].counter).toEqual(1);
    expect(spy.thisValues[0].getCounter()).toEqual(1);
    some.inc();
    expect(spy.thisValues[0].counter).toEqual(2);
    expect(spy.thisValues[0].getCounter()).toEqual(2);
});

It looks like the thisValues[0] object is a proxy and does not refer to the actual instance of some.

This is the error with Sinon 4.2.2:

Expected undefined to equal 1
TypeError: undefined is not a function (evaluating 'spy.thisValues[0].getCounter()')

Reproduce the provided constructor-spy snippet with Sinon 4.2.2 and inspect how constructor calls populate spy.thisValues. Verify that the recorded value is the actual constructed instance, then rerun the snippet and confirm all counter and getCounter assertions pass.