grammar - Why do we say "who you were" and not "whom you were"? Isn't it the object of the verb?

I have a grammar which says that "whom" is used when it follow a preposition.

E.g: to whom am I speaking. to whom it may concern.

The grammar also says that "whom" is the object form of "who".
E.g. He was a person whom everyone regarded as trustworthy. (Whom is the object in the sentence.) "However, this is now felt to be excessively formal by most speakers and who is commonly used instead." (VINCE, M. and SUNDERLAND, P.,2003).

I'm reading "Inferno" and there is this sentence:
"Nobody had any idea who you were (...)"

Wouldn't that be whom? Is "who" in that sentence an object? You is the subject and who would be the object.