Lone leaders entering an assault hex

[campsawyer]

Here is a question to toss around a bit.

A lone leader activates to FIRE and then enters an assault hex where three combat units are engaged but have not yet activated for this turn. The rules state that when "units" enter a assault others can join if not activated. Given that it is just a leader and not a unit, I believe that he cannot start the assault, the combat units need to be with him to start the assault. Thoughts?