fatal:Dimension sizes of left hand side and right hand side of assignment do not match

iam trying to calcultes the means average value of temperature using ICPT data ,after using the cdo command to select years and month files i wrote the scripts following

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/shea_util.ncl"

   diri = "/media/Iomega_Ext_Drive-1/data_regCM/TAS"

   fili = systemfunc("cd "+diri+" ; ls tas*02.nc")
print(fili)    ; these may not be in chronological oerder
   nfil = dimsizes( fili )
;print (nfil)
          do n=0,nfil-1
  PP = addfile( fili(n) ,"r")
  a = PP->tas
aa= a - 273.15
  Mean = dim_avg_n_Wrap(aa,(/0,1/)) 
  b= avg(Mean)
print(b)

end do
but i receive the message
fatal:Dimension sizes of left hand side and right hand side of assignment do not match
what is wrong

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Received on Sat Aug 25 03:06:55 2012