Who or whom?

I'm in the midst of editing a book—as usual!—and ran into a nice little phrase that made me stop and actually mentally diagram the sentence. It contains a few things that make you stop to think. Here's the phrase:

“...remind them of who they are…”

What's the problem, you ask? Shouldn't it just be "whom"? After all, it's the object of the preposition "of," right?

Well, yes and no. The whole phrase "who they are" is the object of the preposition "of" not just "who."

OK, you say, but it still should be "whom" because it is the object of "are," right?

Nope. "Are" is a copulative (linking) verb and takes a subjective case predicate (for those of you with Greek or Latin, a predicate nominative). So, it should be "who" as the subjective complement of "are."

Let's create a different clause with the same construction.

It comes down the to the problem of who is Santa Claus.

So you see, Virginia, there really can be an objective Santa Clause...

I know, it's terrible...