troubles wtih new arrays

[Gavr]

I have a problem with array, which a making in GrADS.
For example, I have U and V arrays and I'm trying to make kinetic
energy:
'define ekin=u*u+v*v'
it works, but I can't make a 'cdiff(ekin)' - it returns:
===========================================================
Data Request Error: Invalid grid coordinates
  World coordinates convert to non-integer grid coordinates
    Variable = ekin  Dimension = 2
  Error ocurred at column 6
DEFINE error:  Invalid expression.
===========================================================
as I noticed, its impossible to make 'd ekin(z+1)' or 'd ekin(z-1)'
its happens only for array, created inside GrADS:
I have no troubles with arrays, described in control-file.
Only with new ones.

Can anyone solve that?
Thank you!