From a strictly grammatical point of view it's no worse than using a plural personal pronoun in the singular in the second person and in fact it's even less incorrect*, since the second person pronoun uses the accusative/dative form in the nominative.

*Assuming that 'incorrect' is a valid term to use for a usage that has been with the language from the git-go. They/their them entered the language in the Middle English period, replacing the native hie/heora/him.