it is I / it is me
Subject vs. object pronoun after it is.
After a linking verb such as is or was, standard English uses the subject pronoun, as in It is I and It was they who called. In conversation, nearly everyone says it’s me and it was them, and it is I can sound stiff. In writing, or whenever the speaker wants to be formal or technically correct, use the subject pronoun.
Examples
UseIt was she who found the error. (she is the subject form)
UseWho’s at the door? It’s me. (everyday conversation)
AvoidIt was her who found the error. (in writing or formal speech)