To be unjust

Mastronarde’s answer key gives

It does not befit a virtuous man to be unjust
as
τῷ ἀγαθῷ οὐ πρέπει ἀδίκῳ εἶναι.

I am curious about that dative ἀδίκῳ. Is it just a mistake? It reminds me of how Russian uses the instrumental in verbal complements. However, I can’t immediately find anything describing such a use in Greek.

If it will help posters, the exercise is from Unit 10, Part IV, Exercise 8, page 90.

This question seemed familiar, so I looked back and saw that I partially asked about this and another question in another thread. One of our veterans stated that πρέπει usually takes a dative. That is as far as this beginner can take this. I defer to the others.

Smyth grammar 1062

  1. A predicate substantive, adjective, or participle referring to a dative stands in the dative or in the accusative in agreement with the unexpressed subject of the infinitive: ““νῦν σοι ἔξεστιν ἀνδρὶ γενέσθαι” now it is in your power to prove yourself a man” X. A. 7.1.21, ““Λακεδαιμονίοις ἔξεστιν ὑ_μῖν φίλους γενέσθαι” it is in your power to become friends to the Lacedaemonians” T. 4.29, ““ἔδοξεν αὐτοῖς . . . ἐξοπλισαμένοις προϊ_έναι” they decided to arm themselves fully and to advance” X. A. 2.1.2, ἔδοξεν αὐτοῖς προφυλακὰ_ς καταστήσαντας συγκαλεῖν τοὺς στρατιώτα_ς they decided to station pickets and to assemble the soldiers 3. 2. 1, ““συμφέρει αὐτοῖς φίλους εἶναι μᾶλλον ἢ πολεμίους” it is for their interest to be friends rather than enemies” X. O. 11.23.
    For predicate nouns in the nominative or accusative in agreement with omitted subject of the infinitive, see 1973-1975.

Thanks guys!

The dative with πρέπει is expected, it’s part of Mastronarde’s lesson. But the cite from Smyth is the answer I was seeking, and would never have found on my own.