question on average

Dear NCL community,

I am using dim_avg_n to do annual mean from monthly mean, e.g.

   ntim=12
   var=new((/ntim,nlat,nlon/), "float")
   do nt=0,ntim-1
   var(nt,:,:) = fbindirread(fname,0, (/nlat,nlon/), "float")
   end do
   var_ave = dim_avg_n(var, 0)

in this function the missing values are ignored. Now I have data sets for 12 months, but for some months there are missing values for some grid points.
I want to set the annual average for these points as a specific value, e.g. zero, rather than to get the average from the available month (as done by dim_avg_n)..

Does anybody tell me how to do that?

Thanks,
Xiaoyan