Actually, it isn't wrong when you say 'een', it's both correct. Maybe without 'een' it's slightly more definite. Like you would be watching the game and you'd say to someone there 'mijn broer is verdediger (of this team)' and when you'd be at school taking about your brother 'mijn broer is een verdediger' is more general, could be of any team. But really, it doesn't matter. it's kind of an unwritten rule, maybe other dutch people don't even see it this way and I'm just over thinking this.