r - Im getting NULL for both key and value in rhadoop code

Question

train.mr <-mapreduce (
 train.hdfs,
 map = function (k, v) {
 keyval (k, v$item)
 },
 reduce = function (k, v) {
 m <-merge (v, v)
 keyval (m$x, m$y)
 }
)

from.dfs(train.mr)

If I try to execute above type of code, I will get

$key
NULL
$val
NULL

Output is generating in rhadoop in a directory "/tmp/file" but Im not able to retrieve by using from.dfs("/tmp/file") If I mention backend = "local", i will get output, but if i give backend = "hadoop", i will get null values for key and value.