Dataframe fails with nested User Defined Types

import org.apache.spark.{SparkConf, SparkContext}
import org.apache.spark.sql.catalyst.InternalRow
import org.apache.spark.sql.catalyst.expressions.GenericMutableRow
import org.apache.spark.sql.types._

@SQLUserDefinedType(udt = classOf[AUDT])
case class A(list:Seq[B])

class AUDT extends UserDefinedType[A] {
  override def sqlType: DataType = StructType(Seq(StructField("list", ArrayType(BUDT, containsNull = false), nullable = true)))
  override def userClass: Class[A] = classOf[A]
  override def serialize(obj: Any): Any = obj match {
    case A(list) =>
      val row = new GenericMutableRow(1)
      row.update(0, new GenericArrayData(list.map(_.asInstanceOf[Any]).toArray))
      row
  }

  override def deserialize(datum: Any): A = {
    datum match {
      case row: InternalRow => new A(row.getArray(0).toArray(BUDT).toSeq)
    }
  }
}

object AUDT extends AUDT

@SQLUserDefinedType(udt = classOf[BUDT])
case class B(text:Int)

class BUDT extends UserDefinedType[B] {
  override def sqlType: DataType = StructType(Seq(StructField("num", IntegerType, nullable = false)))
  override def userClass: Class[B] = classOf[B]
  override def serialize(obj: Any): Any = obj match {
    case B(text) =>
      val row = new GenericMutableRow(1)
      row.setInt(0, text)
      row
  }

  override def deserialize(datum: Any): B = {
    datum match {  case row: InternalRow => new B(row.getInt(0))  }
  }
}

object BUDT extends BUDT

object Test {
  def main(args:Array[String]) = {

    val col = Seq(new A(Seq(new B(1), new B(2))),
      new A(Seq(new B(3), new B(4))))

    val sc = new SparkContext(new SparkConf().setMaster("local[1]").setAppName("TestSpark"))
    val sqlContext = new org.apache.spark.sql.SQLContext(sc)
    import sqlContext.implicits._

    val df = sc.parallelize(1 to 2 zip col).toDF("id","b")
    df.select("b").show()
    df.collect().foreach(println)
  }
}

16/01/18 14:36:22 ERROR Executor: Exception in task 0.0 in stage 0.0 (TID 0)
java.lang.ClassCastException: scala.runtime.BoxedUnit cannot be cast to org.apache.spark.sql.catalyst.InternalRow
	at org.apache.spark.sql.catalyst.expressions.BaseGenericInternalRow$class.getStruct(rows.scala:51)
	at org.apache.spark.sql.catalyst.expressions.GenericMutableRow.getStruct(rows.scala:248)
	at org.apache.spark.sql.catalyst.expressions.GeneratedClass$SpecificUnsafeProjection.apply(Unknown Source)
	at org.apache.spark.sql.execution.Project$$anonfun$1$$anonfun$apply$1.apply(basicOperators.scala:51)
	at org.apache.spark.sql.execution.Project$$anonfun$1$$anonfun$apply$1.apply(basicOperators.scala:49)
	at scala.collection.Iterator$$anon$11.next(Iterator.scala:328)
	at scala.collection.Iterator$$anon$11.next(Iterator.scala:328)
	at scala.collection.Iterator$$anon$11.next(Iterator.scala:328)
	at scala.collection.Iterator$$anon$10.next(Iterator.scala:312)
	at scala.collection.Iterator$class.foreach(Iterator.scala:727)
	at scala.collection.AbstractIterator.foreach(Iterator.scala:1157)
	at scala.collection.generic.Growable$class.$plus$plus$eq(Growable.scala:48)
	at scala.collection.mutable.ArrayBuffer.$plus$plus$eq(ArrayBuffer.scala:103)
	at scala.collection.mutable.ArrayBuffer.$plus$plus$eq(ArrayBuffer.scala:47)
	at scala.collection.TraversableOnce$class.to(TraversableOnce.scala:273)
	at scala.collection.AbstractIterator.to(Iterator.scala:1157)
	at scala.collection.TraversableOnce$class.toBuffer(TraversableOnce.scala:265)
	at scala.collection.AbstractIterator.toBuffer(Iterator.scala:1157)
	at scala.collection.TraversableOnce$class.toArray(TraversableOnce.scala:252)
	at scala.collection.AbstractIterator.toArray(Iterator.scala:1157)
	at org.apache.spark.sql.execution.SparkPlan$$anonfun$5.apply(SparkPlan.scala:212)
	at org.apache.spark.sql.execution.SparkPlan$$anonfun$5.apply(SparkPlan.scala:212)
	at org.apache.spark.SparkContext$$anonfun$runJob$5.apply(SparkContext.scala:1858)
	at org.apache.spark.SparkContext$$anonfun$runJob$5.apply(SparkContext.scala:1858)
	at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:66)
	at org.apache.spark.scheduler.Task.run(Task.scala:89)
	at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:213)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:745)