pronouns - Is “It must be him with whom you enjoy doing your assignments, not me” correct?

I’d like all of you to please consider the following sentence:

It must be him with whom you enjoy doing your assignments, not me.

I have known that after 'to be' verb pronouns words take the subjective form. For example:

It is he who was absent yesterday.

So, can I say that the first sentence is erroneous? Would it be correct to write

It must be he with whom you enjoy doing your assignments, not I.