NPE on vectorization group by

public void copySelected(
    boolean selectedInUse, int[] sel, int size, BytesColumnVector output) {

  // Output has nulls if and only if input has nulls.
  output.noNulls = noNulls;
  output.isRepeating = false;

  // Handle repeating case
  if (isRepeating) {
    output.setVal(0, vector[0], start[0], length[0]);
    output.isNull[0] = isNull[0];
    output.isRepeating = true;
    return;
  }

Caused by: org.apache.hadoop.hive.ql.metadata.HiveException: Hive Runtime Error while processing row
at org.apache.hadoop.hive.ql.exec.vector.VectorMapOperator.process(VectorMapOperator.java:883)
at org.apache.hadoop.hive.ql.exec.tez.MapRecordSource.processRow(MapRecordSource.java:86)
... 17 more
Caused by: java.lang.NullPointerException
at java.lang.System.arraycopy(Native Method)
at org.apache.hadoop.hive.ql.exec.vector.BytesColumnVector.setVal(BytesColumnVector.java:173)
at org.apache.hadoop.hive.ql.exec.vector.BytesColumnVector.copySelected(BytesColumnVector.java:321)
at org.apache.hadoop.hive.ql.exec.vector.expressions.IfExprStringGroupColumnStringGroupColumn.evaluate(IfExprStringGroupColumnStringGroupColumn.java:85)
at org.apache.hadoop.hive.ql.exec.vector.expressions.aggregates.gen.VectorUDAFMaxString.aggregateInputSelection(VectorUDAFMaxString.java:135)
at org.apache.hadoop.hive.ql.exec.vector.VectorGroupByOperator$ProcessingModeBase.processAggregators(VectorGroupByOperator.java:218)
at org.apache.hadoop.hive.ql.exec.vector.VectorGroupByOperator$ProcessingModeHashAggregate.doProcessBatch(VectorGroupByOperator.java:408)
at org.apache.hadoop.hive.ql.exec.vector.VectorGroupByOperator$ProcessingModeBase.processBatch(VectorGroupByOperator.java:179)
at org.apache.hadoop.hive.ql.exec.vector.VectorGroupByOperator.process(VectorGroupByOperator.java:1021)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:897)
at org.apache.hadoop.hive.ql.exec.vector.VectorSelectOperator.process(VectorSelectOperator.java:137)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:897)
at org.apache.hadoop.hive.ql.exec.vector.VectorFilterOperator.process(VectorFilterOperator.java:123)
at org.apache.hadoop.hive.ql.exec.Operator.forward(Operator.java:897)
at org.apache.hadoop.hive.ql.exec.TableScanOperator.process(TableScanOperator.java:130)
at org.apache.hadoop.hive.ql.exec.vector.VectorMapOperator.process(VectorMapOperator.java:783)
... 18 more

CREATE TABLE `test_table`(
 `a` string,
 `b` string,
 `c` string)
 ROW FORMAT SERDE
 'org.apache.hadoop.hive.ql.io.orc.OrcSerde'
STORED AS INPUTFORMAT
 'org.apache.hadoop.hive.ql.io.orc.OrcInputFormat'
 OUTPUTFORMAT
 'org.apache.hadoop.hive.ql.io.orc.OrcOutputFormat'
 TBLPROPERTIES (
 'orc.compress'='SNAPPY',
 'orc.compress.size'='262144',
 'orc.create.index'='true',
 'orc.row.index.stride'='10000',
 'orc.stripe.size'='268435456',
 'transient_lastDdlTime'='1517556432');

SELECT
 NVL(Max(CASE WHEN b IN ('some_literal') THEN b ELSE c END),'') AS scol
FROM test_table
GROUP BY a
LIMIT 1000;