Quote Originally Posted by Soft sign View Post
That’s a really tricky question!

First, there’s no accusative here. The verb быть is special, it cannot have a direct object. Its arguments are either both in nominative, or one in nominative and the other in instrumental.
Моя́ ня́ня (nom) была́ францу́женка (nom).
Моя́ ня́ня (nom) была́ францу́женкой (inst).
Мое́й ня́ней (inst) была́ францу́женка (nom).
Accusative forms (мою́ ня́ню or францу́женку) are impossible here.

The verb был/была́/бы́ло agrees in gender with the argument in nominative. When both arguments are in nominative, it theoretically can agree with either of them:
гла́вный приз был маши́на ~ гла́вный приз была́ маши́на
But these forms are generally rare—when a variant with instrumental can be used, it is preferable:
гла́вным при́зом была́ маши́на.

«Э́то был[а́/о] …» is a special case where the instrumental is impossible.
In this particular case, when the other argument is a noun, the verb agrees in gender with that noun, not with это. When the other argument is not a noun, the neuter gender is used (Э́то бы́ло хорошо́.)
Yeah, that's the answer there! And I think it explains why my first instinct was to take both это and ничья as subjects of the sentence. Great post, Soft Sign!