Pig: error while union the result of mapreduce - hadoop

Question

Pig script
base = load 'u.base' as (uid:long, gid:long, pref:double);
sim1 = mapreduce 'mahout-core-0.7-job.jar'
store base into 'input'
load 'output' as (gid1:long, gid2:long, sim:double)
`org.apache.mahout.cf.taste.hadoop.similarity.item.ItemSimilarityJob -i input -o output -s SIMILARITY_EUCLIDEAN_DISTANCE`;
sim2 = foreach sim1 generate gid2 as gid1, gid1 as gid2, sim;
sim3 = union sim1,sim2;
dump sim3;
Pig output
2013-03-28 09:21:32,564 [main] INFO org.apache.pig.tools.pigstats.ScriptState - Pig features used in the script: UNION,NATIVE
2013-03-28 09:21:32,676 [main] INFO org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MRCompiler - File concatenation threshold: 100 optimistic? false
2013-03-28 09:21:32,699 [main] INFO org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MultiQueryOptimizer - MR plan size before optimization: 4
2013-03-28 09:21:32,702 [main] ERROR org.apache.pig.tools.grunt.Grunt - ERROR 2127: Internal Error: Cloning of plan failed for optimization.
Details at logfile: /home/chenwl/logs/pig_1364433685680.log
Pig log
Pig Stack Trace
---------------
ERROR 2127: Internal Error: Cloning of plan failed for optimization.
org.apache.pig.impl.logicalLayer.FrontendException: ERROR 1066: Unable to open iterator for alias sim3
at org.apache.pig.PigServer.openIterator(PigServer.java:836)
at org.apache.pig.tools.grunt.GruntParser.processDump(GruntParser.java:696)
at org.apache.pig.tools.pigscript.parser.PigScriptParser.parse(PigScriptParser.java:320)
at org.apache.pig.tools.grunt.GruntParser.loadScript(GruntParser.java:531)
at org.apache.pig.tools.grunt.GruntParser.processScript(GruntParser.java:480)
at org.apache.pig.tools.pigscript.parser.PigScriptParser.Script(PigScriptParser.java:804)
at org.apache.pig.tools.pigscript.parser.PigScriptParser.parse(PigScriptParser.java:449)
at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:194)
at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:170)
at org.apache.pig.tools.grunt.Grunt.run(Grunt.java:69)
at org.apache.pig.Main.run(Main.java:538)
at org.apache.pig.Main.main(Main.java:157)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.hadoop.util.RunJar.main(RunJar.java:156)
Caused by: org.apache.pig.PigException: ERROR 1002: Unable to store alias sim3
at org.apache.pig.PigServer.storeEx(PigServer.java:935)
at org.apache.pig.PigServer.store(PigServer.java:898)
at org.apache.pig.PigServer.openIterator(PigServer.java:811)
... 16 more
Caused by: org.apache.pig.impl.plan.optimizer.OptimizerException: ERROR 2127: Internal Error: Cloning of plan failed for optimization.
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MultiQueryOptimizer.mergeDiamondMROper(MultiQueryOptimizer.java:304)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MultiQueryOptimizer.visitMROp(MultiQueryOptimizer.java:219)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MapReduceOper.visit(MapReduceOper.java:273)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MapReduceOper.visit(MapReduceOper.java:46)
at org.apache.pig.impl.plan.ReverseDependencyOrderWalker.walk(ReverseDependencyOrderWalker.java:71)
at org.apache.pig.impl.plan.PlanVisitor.visit(PlanVisitor.java:46)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MultiQueryOptimizer.visit(MultiQueryOptimizer.java:94)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MapReduceLauncher.compile(MapReduceLauncher.java:617)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MapReduceLauncher.launchPig(MapReduceLauncher.java:146)
at org.apache.pig.PigServer.launchPlan(PigServer.java:1264)
at org.apache.pig.PigServer.executeCompiledLogicalPlan(PigServer.java:1249)
at org.apache.pig.PigServer.storeEx(PigServer.java:931)
... 18 more
Caused by: java.lang.CloneNotSupportedException: Unable to find clone for op 1-36: Native('hadoop jar mahout-core-0.7-job.jar org.apache.mahout.cf.taste.hadoop.similarity.item.ItemSimilarityJob -i input -o output -s SIMILARITY_EUCLIDEAN_DISTANCE ') - scope-12
at org.apache.pig.backend.hadoop.executionengine.physicalLayer.plans.PhysicalPlan.clone(PhysicalPlan.java:273)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MultiQueryOptimizer.mergeDiamondMROper(MultiQueryOptimizer.java:298)
... 29 more
================================================================================
Environment
OS: ubuntu 12.04
Hadoop: 1.0.4 Subversion https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1.0 -r 1393290
Pig: 0.11.0 (r1446324)
P.S.:
It works if sim1 was loaded from hdfs, e.g. sim1 = load 'sim' as (gid1:long, gid2:long, sim:double).