tryWrite does not merge root and newData

[ibash]

Given rules like this:

{
  "rules": {
    "foo": {
      "id": {
        ".validate": "newData.isString()"
      },
      "bar": {
        ".validate": "newData.isString()"
      }
    }
  }
}

Shouldn't the tryWrite pass? In a similar test I'm seeing a rejection at the level of /foo with newData.hasChildren[['id', 'bar'])

targaryen.setFirebaseData({foo: {id: 'hi', bar: 'blah'}})
root = targaryenHelpers.getFirebaseData()
rules = targaryenHelpers.getFirebaseRules()
rules.tryWrite('foo/bar', root, 'taco', ...)