KinesisIO.write causes NPE as the producer is null

org.apache.beam.runners.flink.translation.wrappers.streaming.io.UnboundedSourceWrapper.run(UnboundedSourceWrapper.java:297)\n\tat org.apache.flink.streaming.api.operators.StreamSource.run(StreamSource.java:93)\n\tat org.apache.flink.streaming.api.operators.StreamSource.run(StreamSource.java:57)\n\tat org.apache.flink.streaming.runtime.tasks.SourceStreamTask.run(SourceStreamTask.java:97)\n\tat org.apache.flink.streaming.runtime.tasks.StoppableSourceStreamTask.run(StoppableSourceStreamTask.java:45)\n\tat org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:300)\n\tat org.apache.flink.runtime.taskmanager.Task.run(Task.java:711)\n\tat java.lang.Thread.run(Thread.java:748)\nCaused by: java.lang.NullPointerException: null\n\tat org.apache.beam.sdk.io.kinesis.KinesisIO$Write$KinesisWriterFn.flushBundle(KinesisIO.java:685)\n\tat org.apache.beam.sdk.io.kinesis.KinesisIO$Write$KinesisWriterFn.finishBundle(KinesisIO.java:669)