Lineage fail when flatten a bag

The following script fail:

a = load '1.txt' as (a0);
b = foreach a generate flatten((bag{tuple(map[])})a0) as b0:map[];
c = foreach b generate (long)b0#'key1';
dump c;

1.txt:

Error message:
org.apache.pig.backend.executionengine.ExecException: ERROR 1075: Received a bytearray from the UDF. Cannot determine how to convert the bytearray to long.
at org.apache.pig.backend.hadoop.executionengine.physicalLayer.expressionOperators.POCast.getNext(POCast.java:286)
at org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.processPlan(POForEach.java:361)
at org.apache.pig.backend.hadoop.executionengine.physicalLayer.relationalOperators.POForEach.getNext(POForEach.java:291)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigMapBase.runPipeline(PigMapBase.java:236)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigMapBase.map(PigMapBase.java:231)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigMapBase.map(PigMapBase.java:53)
at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:144)
at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:621)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:305)
at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:177)

In logical plan:

fake: Store 1-30 Schema: {long} Type: Unknown
|
|---c: ForEach 1-29 Schema: {long} Type: bag
    |   |
    |   Cast 1-28--null FieldSchema: long Type: long
    |   |
    |   |---MapLookup 1-26 FieldSchema: bytearray Type: bytearray
    |       |
    |       |---Project 1-27 Projections: [0] Overloaded: false FieldSchema: b0: map Type: map
    |           Input: b: ForEach 1-21
    |
    |---b: ForEach 1-21 Schema: {b0: map[ ]} Type: bag
        |   |
        |   Cast 1-20--org.apache.pig.builtin.PigStorage FieldSchema: a0: bag({(map[ ])}) Type: bag
        |   |
        |   |---Project 1-19 Projections: [0] Overloaded: false FieldSchema: a0: bytearray Type: bytearray
        |       Input: a: Load 1-15
        |
        |---a: Load 1-15 Schema: {a0: bytearray} Type: bag

Cast 1-28 get null caster. This error cannot be fixed by -Dusenewlogicalplan=false flag.