Dtrace failing to match probes of provider Ruby

Hello everyone,
I am using Ruby 2.0 in Ubuntu 11.04.
When I try to get probes provided by the Ruby provider using “dtrace -l
-P ruby”, I’m getting a msg saying “dtrace: failed to match ruby:::: No
probe matches description”.
I installed Ruby using apt-get.
Why I can’t use ruby probes here?
How can I fix this?