fix rigid

Hi,
I have simulation box filled with particles, I want to move cluster of some particles in this box. To do so, i defined one spherical region and then apply following command:
....
....
region mySphere sphere .........
group myGroup region mySphere
fix 1 myRigid rigid group 1 myGroup
...
...

Then I want to move this as spherical body made of particle in the simulation box consists of other particles.
Is this right way of doing this? When i run the simulation then it prompts:

447 atoms in group myGroup
ERROR: One or zero atoms in rigid body

Is there something wrong with my script?
Thanks in advance...
Ram