java.lang.NullPointerException when save as TFRecords

[SoloBean]

Caused by: java.lang.NullPointerException at org.tensorflow.spark.datasources.tfrecords.serde.DefaultTfRecordRowEncoder$.$anonfun$encodeFeature$5(DefaultTfRecordRowEncoder.scala:136) at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:238) at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:36) at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33) at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:198) at scala.collection.TraversableLike.map(TraversableLike.scala:238) at scala.collection.TraversableLike.map$(TraversableLike.scala:231) at scala.collection.mutable.ArrayOps$ofRef.map(ArrayOps.scala:198) at org.tensorflow.spark.datasources.tfrecords.serde.DefaultTfRecordRowEncoder$.encodeFeature(DefaultTfRecordRowEncoder.scala:136) at org.tensorflow.spark.datasources.tfrecords.serde.DefaultTfRecordRowEncoder$.$anonfun$encodeExample$1(DefaultTfRecordRowEncoder.scala:64) at scala.collection.immutable.List.foreach(List.scala:392) at org.tensorflow.spark.datasources.tfrecords.serde.DefaultTfRecordRowEncoder$.encodeExample(DefaultTfRecordRowEncoder.scala:61) at org.tensorflow.spark.datasources.tfrecords.DefaultSource.$anonfun$createRelation$3(DefaultSource.scala:61) at scala.collection.Iterator$$anon$10.next(Iterator.scala:459) at org.apache.spark.internal.io.SparkHadoopWriter$.$anonfun$executeTask$1(SparkHadoopWriter.scala:132) at org.apache.spark.util.Utils$.tryWithSafeFinallyAndFailureCallbacks(Utils.scala:1452) at org.apache.spark.internal.io.SparkHadoopWriter$.executeTask(SparkHadoopWriter.scala:130) ... 9 more

what is data like can case this exception?