Error in clusterR

Hi,

I wrote a code which uses clusterR to run a function on multiple CPUs to 
process MODIS 16 day dataset. The function converts a brick to 
dataframe. The existing commands to do this in package raster is very 
slow on large datasets, therefore I'm attempting to use the multicore 
option to speed up the process.

I get the following error
Error in clusterR(n1, qckextract) : cluster error

I would appreciate your advice on to fix the function. Thanks!

library(raster)

n1<-brick("ndvih25v08masked.grd")

beginCluster(nice=20)
nodes <- length(getCluster())
bs <- blockSize(n1, minblocks=nodes*4)

qckextract <- function(i) {
   for (i in 1:length(bs$n)) {
     e<- extent(xmin(x), xmax(x), yFromRow(x, bs$row[i]+bs$nrows[i]-1) - 
0.5 * yres(x), yFromRow(x, bs$row[i])+0.5 * yres(x))
     n <- cellsFromExtent(x,e)
     v <- (getValues(x, bs$row[i], bs$nrows[i]))/10000
     v<-as.data.frame(v)
     v<- as.data.frame(cbind(n,v))
   }
   return(v)
}

nx<-clusterR(n1,qckextract)
endCluster()