Cron task output is 0, script output is OK

I have the following cron task set to run every 15 minutes to ascertain how many users are in the system and append the result to the log.

/home/pronto/cus/whoisinc >> /home/pronto/cus/whoisin.log

This is the whoisinc script

date +"%d-%m-%Y,%k:%M,Pronto Users,`prowho -s | grep -v SESSION | wc -l`"

Running it manually to screen I get the following which is what I want
20-10-2010, 8:55,Pronto Users,31
When cron task runs it, I get 0 every time
20-10-2010, 9:15,Pronto Users,0

What am I doing wrong? Can anyone help me?

Depending on your OS when you run your script by cron, you may loose some environment variables.
You have to make sure that you take care to load them from your script

. /home/myuser/.profile

(check that /home/pronto/cus is into your PATH env variable)