immutable.js “invalid” setIn behavior

const { fromJS } = require('immutable@4.0.0-rc.12'); const original = fromJS({ foo: { items: ['zero', 'one', 'two'], }, }); let modified = original.setIn(['foo', 'items', 2], 'dos'); console.log(modified); modified = modified.setIn(['foo', 'bar'], 'snickers'); console.log(modified); modified = modified.setIn(['foo', 'items', 'baz'], 'qux'); // this update is ignored console.log('final:', modified);