Trying to add an URL alias to the site root but disabling the "Place alias at the site root" results in a "Could not find 'URLAlias' with identifier '1'" error.

Steps to reproduce:
1. Go to "Content" -> "Content structure"
2. Go to "URL" tab (you should be viewing the root node)
3. Click the Plus sign
4. Enter URL and disable the "Place alias at the site root" checkbox.
5. Click Create

Actual:
Error page (404) with message:

Could not find 'URLAlias' with identifier '1'

is displayed

Expected:
The checkbox "Place alias at the site root" is disabled for root? It does not make sense there really.