What's the call?

[shocker]

SO let's say that a player on B went to the sideline and is being replaced by a substitute. The substitute is heading to the defensive side of the ball but has not made it accross the nuetral zone before the snap. That player is then still a substitute so it cannot be an encroachment foul.

I don't have my rule book with me. I am assuming this play would be killed and an illegal substitution foul called?