Querying a single instance of metric

[Chandana De Silva <chandana@xxxxxxxxxxxxx>]

Hello All

I am trying to query a single instance of a metric which is visible when 
you query all instances:
[chandana@tardis tmp]$ pminfo -f -a 20120327 kernel.percpu.cpu.idle

kernel.percpu.cpu.idle
    inst [0 or "cpu0"] value 63792152140
    inst [1 or "cpu1"] value 69386074790
    inst [2 or "cpu2"] value 73923012890
    inst [3 or "cpu3"] value 67765150660

But not when you query one instance.

[chandana@tardis tmp]$ pminfo -f -a 20120327 kernel.percpu.cpu.idle[0]
Error: kernel.percpu.cpu.idle[0]: Unknown metric name

Why doesn't this one work ? I want to be able to list a single instance. 
I am sure that I have doe this before .. It is the same on all the 
tools. What am I doing wrong ?
It makes no difference whether it is an archive or not, this on my box:

[chandana@tardis ~]$ pminfo -f kernel.percpu.cpu.user

kernel.percpu.cpu.user
    inst [0 or "cpu0"] value 1253780
    inst [1 or "cpu1"] value 574040

[chandana@tardis ~]$ pminfo -f kernel.percpu.cpu.user[0]
Error: kernel.percpu.cpu.user[0]: Unknown metric name

Thanks,
Chandana