Maximum z-cooridnate(height) of pile in a region

Hi
I would like to find the Maximum z coordinate of particles pile in a region so i tried using
compute zMax all reduce/region region-Id max z
variable zmax equal c_zMax
print "Height of pile = ${zmax}"
i tried by placing run 0 also and giving compute id in thermo style and update_on_run_end also in compute id and how many times i tried it showing me same error
"compute in variable between runs is not current. try using update_on_run_end"
Can anyone help me with this problem
Regards,
Kavya