problem about function dpres_plevel

Hi, Everyone
     I'd like to get the thickness of pressure levels with dpres_plevel
function , and found if I did
it from top atmosphere to surface , all the thicknesses I got are zero (See
my testing script).
Does any one know how I can make that? Is it right I only need to add a
"-"sign in my integration term?
Thanks for any of your suggestion.

     Yan

  ; PRESSURE (hPa)
  plev = (/ 1000.,950.,900.,850.,800.,750.,700.,650.,600., \
             550.,500.,450.,400.,350.,300.,250.,200. /)
  plev = plev*100.
  plev@units = "Pa" ; Pa = kg/(m s2) (Pascal)
  ptop = *min*
<http://www.ncl.ucar.edu/Document/Functions/Built-in/min.shtml>(plev)
  psfc = 1018*100.
  psfc@units = "Pa"

  dp = dpres_plevel(plev,psfc,ptop);

  rdp=dpres_plevel(plev,psfc,ptop); ==0

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Received on Mon Nov 8 11:59:47 2010