The first time a client connects to the cluster and sends a bunch of inserts in parallel I get this assertion error. All subsequent inserts (including just retrying the first requests) work fine. The only assert in PrecisionTime is checking if the millis argument is <= the current millis. What could cause this to not be true, and what can I do to avoid/fix it?

java.lang.AssertionError: null
at org.apache.cassandra.cql3.Lists$PrecisionTime.getNext(Lists.java:275) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.cql3.Lists$Prepender.execute(Lists.java:430) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.cql3.statements.UpdateStatement.addUpdateForKey(UpdateStatement.java:94) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.cql3.statements.ModificationStatement.addUpdates(ModificationStatement.java:682) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.cql3.statements.ModificationStatement.getMutations(ModificationStatement.java:613) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.cql3.statements.ModificationStatement.executeWithoutCondition(ModificationStatement.java:420) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.cql3.statements.ModificationStatement.execute(ModificationStatement.java:408) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.cql3.QueryProcessor.processStatement(QueryProcessor.java:206) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.cql3.QueryProcessor.processPrepared(QueryProcessor.java:487) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.cql3.QueryProcessor.processPrepared(QueryProcessor.java:464) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.transport.messages.ExecuteMessage.execute(ExecuteMessage.java:130) ~[apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.transport.Message$Dispatcher.channelRead0(Message.java:507) [apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.transport.Message$Dispatcher.channelRead0(Message.java:401) [apache-cassandra-3.0.8.jar:3.0.8]
at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105) [netty-all-4.0.23.Final.jar:4.0.23.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:333) [netty-all-4.0.23.Final.jar:4.0.23.Final]
at io.netty.channel.AbstractChannelHandlerContext.access$700(AbstractChannelHandlerContext.java:32) [netty-all-4.0.23.Final.jar:4.0.23.Final]
at io.netty.channel.AbstractChannelHandlerContext$8.run(AbstractChannelHandlerContext.java:324) [netty-all-4.0.23.Final.jar:4.0.23.Final]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [na:1.8.0_101]
at org.apache.cassandra.concurrent.AbstractLocalAwareExecutorService$FutureTask.run(AbstractLocalAwareExecutorService.java:164) [apache-cassandra-3.0.8.jar:3.0.8]
at org.apache.cassandra.concurrent.SEPWorker.run(SEPWorker.java:105) [apache-cassandra-3.0.8.jar:3.0.8]
at java.lang.Thread.run(Thread.java:745) [na:1.8.0_101]