NullPointerException with in memory BTreeMap on concurrent writes

With a code structure like:

try (DB db = DBMaker.memoryDB().make()) {
      BTreeMap<Long, Integer> tempRevisionTopicOutput = db.treeMap("test").keySerializer(Serializer.LONG).valueSerializer(Serializer.INTEGER).createOrOpen();
     stream..parallel().forEach(data -> {
         //Some computation
         test.put(a, b);
     });
}

I get the following exception:

GRAVE: null
java.lang.NullPointerException
        at org.mapdb.volume.ByteArrayVol.getSlice(ByteArrayVol.java:65)
        at org.mapdb.volume.ByteArrayVol.getLong(ByteArrayVol.java:237)
        at org.mapdb.StoreDirect.getIndexVal(StoreDirect.kt:128)
        at org.mapdb.StoreDirect.updateProtected(StoreDirect.kt:642)
        at org.mapdb.StoreDirect.update(StoreDirect.kt:634)
        at org.mapdb.BTreeMap.put2(BTreeMap.kt:408)
        at org.mapdb.BTreeMap.put(BTreeMap.kt:292)

Environment:

openjdk version "1.8.0_171"
OpenJDK Runtime Environment (build 1.8.0_171-8u171-b11-0ubuntu0.16.04.1-b11)
OpenJDK 64-Bit Server VM (build 25.171-b11, mixed mode)

Start by reproducing the parallel-write example and follow the stack through BTreeMap.kt, StoreDirect.kt, and ByteArrayVol.java, beginning at ByteArrayVol.getSlice. Done means concurrent writes to the in-memory BTreeMap no longer produce the reported NullPointerException.