annual mean and contour

Hi

I want to calculate the annual mean from 12 months hdf files, and then make a contour, below is what I did.
There is a fatal error as below
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fatal:Coordinate variables must be the same dimension as their dimension
fatal:Execute: Error occurred at or near line 43 in file
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line 43 is "   var&lat        = ispan(-42,42,1)*dlat "

Also the values of var is not correct. can anyone help to fix the problem?

Thanks,
Xiaoyan

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   nlat  = 85
   nlon  = 72
   ntim = 12

   do nt=0,ntim-1
   fname = m+sprinti("%0.2i", nt+1)+".hdf"
   print(fname)
   in = addfiles(fname, "r") ;; load all the files
   var0 = in[:]->variable_name
   end do
   var = dim_avg_n(var0, 0)

   dlat  = 170./nlat
   dlon  = 360./nlon

   var!0          = "lat"
   var&lat        = ispan(-42,42,1)*dlat
   var&lat@units  = "degrees_north"

   var!1          = "lon"
   var&lon        = ispan(-36,35,1)*dlon
   var&lon@units  = "degrees_east"

...