Passing null to child() in Firebase Database Rules

The method .child(), documented here, in the Firebase Database security rules may be passed a value from else where in the database as such.

{
  ".read": "newData.child('someField')
                   .child(root.child('someFieldInDb').val()).val() === 'someValue'"
}

What happens when someFieldInDb has no value and returns null. Essentially what happens when you pass null to .child()? I imagine some exception is thrown, does that mean the rule fails? If so, is this acceptable logic?