No biggie, but when closing a question as exact duplicate, entering a question ID and then deleting it, the following situation may occur:
I have emptied the question text field but still the close button stays enabled. Clicking it will in fact result in a close vote being cast – but apparently without dedicated duplicate question ID. At least, that’s what happened when Jon Seigel tried it on this question. His close vote was counted but when I then tried to close the question as exact duplicate, the list of “master questions” was still empty.
And a the request of Jon Seigel, here's the state of the vote to close dialog on this question:
And here's what the question looked like as a closed question: