What problem does Socrates point out with Polemarchus’ claim that justice is doing good to your friends and harm to your enemies?

Socrates points out that our judgment concerning friends and enemies isn't foolproof, and that taking this stance leads us to harm the good and help the bad. Unless we choose our friends according to their virtues (assuming they are in fact virtuous) and our enemies based upon evil deeds, justice cannot work in this context. Everyone has less than perfect friends, as well as having enemies or people we don't care for, who are good and virtuous people.