Can't set watchpoint

I’m running ruby 1.8.6 on Ubuntu Hardy.
I have rdebug 0.10.3

I start rdebug on a script and try to issue a watch command such as

 watch n == 4

I immediately get the error
NoMethodError Exception: undefined method `watch’ for

I then issue the command “help”. “watch” is not shown on the command

Can anyone offer some help.