runtime error: Cannot read null.client.
proc name: Who (/mob/verb/Who)
usr: SuperAshing (/mob/parm)
src: SuperAshing (/mob/parm)
call stack:
SuperAshing (/mob/parm): Who()

That is the error I keep getting with my Who verb.
The code for my Who verb looks like this:

mob/verb/Who()
set category = "Communication"
var/mob/M
for(M as mob in world)
if(M.client)
usr << "/red Hunters"

I have looked at this for a long while, but I just can't figure out the problem, it would be really great if someone could help me with this one