groupBy on a Dataset of Maps

[Punit Naik]

Below is my code:

val env = ExecutionEnvironment.getExecutionEnvironment
val data=env.readTextFile("file:///home/punit/test").flatMap( line =>
JSON.parseFull(line) )
val j=data.flatMap{ _ match {case map: Map[String, Any] =>
                            {List(Map("ga_date" ->
map.get("ga_dateHour").get.toString().substring(0,
map.get("ga_dateHour").get.toString().length()-2))) }}}

val k=j.groupBy(_.get("ga_date"))

But when I execute this, it throws an exception saying:

org.apache.flink.api.common.InvalidProgramException: Return type
Option[String] of KeySelector class
org.apache.flink.api.scala.DataSet$$anon$12 is not a valid key type

Where am I going wrong?
-- 
Thank You

Regards

Punit Naik