setIn creates extra array in certain situation, w strange behavior

[marbemac]

setIn seems to create an array, if setting an object property on an array that already exists. Although it seems like it's an array, that's an object? Or something of the sort, I'm not sure, I've never seen this before... it's odd. Screenshots to show what I mean

var x = Immutable({foo: []})
x.setIn(['foo', 0, 'bar'], 'val')

// results in
{
    foo: [
        [ ... array object thing?
            bar: val
        ]
    ]
}

When stringified, it's as if setIn never happened...

Confirmed in Chrome 52.0.2743.116, OS X, latest seamless-immutable.