CassandraDaemon.java:167 NullPointerException

ERROR [CompactionExecutor:40] 2015-04-05 18:34:36,413 CassandraDaemon.java:167 - Exception in thread Thread[CompactionExecutor:40,1,main]
java.lang.NullPointerException: null
	at org.apache.cassandra.service.CacheService$KeyCacheSerializer.serialize(CacheService.java:475) ~[apache-cassandra-2.1.3.jar:2.1.3]
	at org.apache.cassandra.service.CacheService$KeyCacheSerializer.serialize(CacheService.java:463) ~[apache-cassandra-2.1.3.jar:2.1.3]
	at org.apache.cassandra.cache.AutoSavingCache$Writer.saveCache(AutoSavingCache.java:274) ~[apache-cassandra-2.1.3.jar:2.1.3]
	at org.apache.cassandra.db.compaction.CompactionManager$11.run(CompactionManager.java:1152) ~[apache-cassandra-2.1.3.jar:2.1.3]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[na:1.8.0_11]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[na:1.8.0_11]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) ~[na:1.8.0_11]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_11]
	at java.lang.Thread.run(Thread.java:745) [na:1.8.0_11]
INFO  [ScheduledTasks:1] 2015-04-05 18:34:38,392 ColumnFamilyStore.java:877 - Enqueuing flush of sstable_activity: 35890 (0%) on-heap, 0 (0%) off-heap
INFO  [MemtableFlushWriter:30] 2015-04-05 18:34:38,401 Memtable.java:339 - Writing Memtable-sstable_activity@388081044(3726 serialized bytes, 1656 ops, 0%/0% of on/off-heap limit)