exception happens when serializing union with identityref

identityref requires special handling during serializing/deserializing.
It is found by neutron northbound it. The log is attached.

yang model.
module neutron-fwaas {
...
grouping firewall-rule-attributes {
leaf protocol {
description "the protocol this rule applies to";
type union {
type uint8;
type identityref

}
}
...
}

exception:
2016-05-24 01:32:13,840 | ERROR | lt-dispatcher-24 | Shard | 201 - org.opendaylight.controller.sal-akka-raft - 1.4.0.SNAPSHOT | member-1-shard-default-config An exception occurred while preCommitting transaction StringIdentifier

java.lang.IllegalArgumentException: Unknown value type Class
at org.opendaylight.controller.cluster.datastore.node.utils.stream.ValueTypes.getSerializableType(ValueTypes.java:83)[200:org.opendaylight.controller.sal-clustering-commons:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.node.utils.stream.AbstractNormalizedNodeDataOutput.writeObject(AbstractNormalizedNodeDataOutput.java:411)[200:org.opendaylight.controller.sal-clustering-commons:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.node.utils.stream.AbstractNormalizedNodeDataOutput.leafNode(AbstractNormalizedNodeDataOutput.java:160)[200:org.opendaylight.controller.sal-clustering-commons:1.4.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.wasProcessAsSimpleNode(NormalizedNodeWriter.java:152)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.write(NormalizedNodeWriter.java:106)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.writeChildren(NormalizedNodeWriter.java:173)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter$OrderedNormalizedNodeWriter.writeMapEntryNode(NormalizedNodeWriter.java:284)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.wasProcessedAsCompositeNode(NormalizedNodeWriter.java:210)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.write(NormalizedNodeWriter.java:102)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.writeChildren(NormalizedNodeWriter.java:173)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.wasProcessedAsCompositeNode(NormalizedNodeWriter.java:240)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.write(NormalizedNodeWriter.java:102)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.writeChildren(NormalizedNodeWriter.java:173)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.wasProcessedAsCompositeNode(NormalizedNodeWriter.java:198)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.write(NormalizedNodeWriter.java:102)[119:org.opendaylight.yangtools.yang-data-api:1.0.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.node.utils.stream.AbstractNormalizedNodeDataOutput.writeNormalizedNode(AbstractNormalizedNodeDataOutput.java:151)[200:org.opendaylight.controller.sal-clustering-commons:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataTreeCandidatePayload.writeNode(DataTreeCandidatePayload.java:88)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataTreeCandidatePayload.writeChildren(DataTreeCandidatePayload.java:60)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataTreeCandidatePayload.writeNode(DataTreeCandidatePayload.java:84)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataTreeCandidatePayload.writeChildren(DataTreeCandidatePayload.java:60)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataTreeCandidatePayload.create(DataTreeCandidatePayload.java:118)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.Shard.continueCommit(Shard.java:334)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.ShardCommitCoordinator.doCommit(ShardCommitCoordinator.java:398)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.ShardCommitCoordinator.doCanCommit(ShardCommitCoordinator.java:357)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.ShardCommitCoordinator.handleCanCommit(ShardCommitCoordinator.java:311)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.ShardCommitCoordinator.handleReadyLocalTransaction(ShardCommitCoordinator.java:255)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.Shard.handleReadyLocalTransaction(Shard.java:523)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.Shard.handleNonRaftCommand(Shard.java:236)[205:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.raft.RaftActor.handleCommand(RaftActor.java:301)[201:org.opendaylight.controller.sal-akka-raft:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.common.actor.AbstractUntypedPersistentActor.onReceiveCommand(AbstractUntypedPersistentActor.java:29)[200:org.opendaylight.controller.sal-clustering-commons:1.4.0.SNAPSHOT]
at akka.persistence.UntypedPersistentActor.onReceive(PersistentActor.scala:165)[194:com.typesafe.akka.persistence:2.4.4]
at org.opendaylight.controller.cluster.common.actor.MeteringBehavior.apply(MeteringBehavior.java:97)[200:org.opendaylight.controller.sal-clustering-commons:1.4.0.SNAPSHOT]
at akka.actor.ActorCell$$anonfun$become$1.applyOrElse(ActorCell.scala:544)[188:com.typesafe.akka.actor:2.4.4]
at akka.actor.Actor$class.aroundReceive(Actor.scala:482)[188:com.typesafe.akka.actor:2.4.4]
at akka.persistence.UntypedPersistentActor.akka$persistence$Eventsourced$$super$aroundReceive(PersistentActor.scala:163)[194:com.typesafe.akka.persistence:2.4.4]
at akka.persistence.Eventsourced$$anon$1.stateReceive(Eventsourced.scala:606)[194:com.typesafe.akka.persistence:2.4.4]
at akka.persistence.Eventsourced$class.aroundReceive(Eventsourced.scala:176)[194:com.typesafe.akka.persistence:2.4.4]
at akka.persistence.UntypedPersistentActor.aroundReceive(PersistentActor.scala:163)[194:com.typesafe.akka.persistence:2.4.4]
at akka.actor.ActorCell.receiveMessage(ActorCell.scala:526)[188:com.typesafe.akka.actor:2.4.4]
at akka.actor.ActorCell.invoke(ActorCell.scala:495)[188:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)[188:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.Mailbox.run(Mailbox.scala:224)[188:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.Mailbox.exec(Mailbox.scala:234)[188:com.typesafe.akka.actor:2.4.4]
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)[184:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)[184:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)[184:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)[184:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]