Fill role in Draft mode

So why is it when I am playing with a 3 or 4 man group in Draft mode it says "not enough diverse role selections" when i pick the "Fill" role and yet if i switch it to 2 roles without any one on my team switching it allows us to queue. Is the "fill" role NOT "ill play any role please"? because it seems really counter intuitive to deny people the ability to fill especially when in large pre mades where you can easily stack the system to get what roles you want