Seeing repeated Uncaught exception on thread Thread[SharedPool-Worker-1,5,main]: {} java.lang.AssertionError: null in logs

WARN  [SharedPool-Worker-1] 2015-10-29 19:52:33,703  AbstractTracingAwareExecutorService.java:169 - Uncaught exception on thread Thread[SharedPool-Worker-1,5,main]: {}
java.lang.AssertionError: null
	at org.apache.cassandra.db.AbstractNativeCell.checkPosition(AbstractNativeCell.java:585) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.AbstractNativeCell.getByteBuffer(AbstractNativeCell.java:657) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.AbstractNativeCell.get(AbstractNativeCell.java:304) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.AbstractNativeCell.get(AbstractNativeCell.java:291) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.AbstractNativeCell.sizeOf(AbstractNativeCell.java:132) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.AbstractNativeCell.<init>(AbstractNativeCell.java:120) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.NativeCell.<init>(NativeCell.java:40) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.utils.memory.NativeAllocator.clone(NativeAllocator.java:72) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.NativeCell.localCopy(NativeCell.java:64) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.AtomicBTreeColumns$ColumnUpdater.apply(AtomicBTreeColumns.java:445) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.AtomicBTreeColumns$ColumnUpdater.apply(AtomicBTreeColumns.java:418) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.utils.btree.NodeBuilder.addNewKey(NodeBuilder.java:322) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.utils.btree.NodeBuilder.update(NodeBuilder.java:190) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.utils.btree.Builder.update(Builder.java:74) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.utils.btree.BTree.update(BTree.java:186) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.AtomicBTreeColumns.addAllWithSizeDelta(AtomicBTreeColumns.java:225) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.Memtable.put(Memtable.java:210) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.ColumnFamilyStore.apply(ColumnFamilyStore.java:1229) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.Keyspace.apply(Keyspace.java:400) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.Keyspace.apply(Keyspace.java:363) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.Mutation.apply(Mutation.java:214) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.db.MutationVerbHandler.doVerb(MutationVerbHandler.java:54) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.net.MessageDeliveryTask.run(MessageDeliveryTask.java:64) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[na:1.8.0_60]
	at org.apache.cassandra.concurrent.AbstractTracingAwareExecutorService$FutureTask.run(AbstractTracingAwareExecutorService.java:164) ~[cassandra-all-2.1.8.689.jar:2.1.8.689]
	at org.apache.cassandra.concurrent.SEPWorker.run(SEPWorker.java:105) [cassandra-all-2.1.8.689.jar:2.1.8.689]
	at java.lang.Thread.run(Unknown Source) [na:1.8.0_60]