So I'm practicing with my Russian friend and for HW she wrote:

Я хочу стать _______ (инженер) , буду целый день собирать ________ (робот)

The answer according to her is:

Я хочу стать инженером, буду целый день собирать роботов.

I understand инженером, but why use the Genitive case in роботов? Nothing would indicate here ownership, "of", "some", "any"....

She, as a native speaker, says it just sounds correct to her. She isn't familiar with cases and grammar names of stuff, so she can't give me the logic.

Personally I thought it's the nominative because robots is the subject and I don't see anything special.

Therefor it ought to be роботи


Anyone?