Problem in simulation

I'm currently using Bonita 6.4, My process model is working fine but the simulation woudn't run. In monitor log I get this error: !ERROR java.lang.NullPointerException !STACK 0 line #1: net.sf.csv4j.ProcessingException: java.lang.NullPointerException at net.sf.csv4j.CSVStreamProcessor.processStream(CSVStreamProcessor.java:153) at net.sf.csv4j.CSVFileProcessor.processFile(CSVFileProcessor.java:127) at org.bonitasoft.simulation.reporting.CSVSimReportStorage.getStoredProcessInstances(CSVSimReportStorage.java:111) at org.bonitasoft.simulation.engine.SimulationEngine.initTodoList(SimulationEngine.java:248) at org.bonitasoft.simulation.engine.SimulationEngine.start(SimulationEngine.java:139) at org.bonitasoft.studio.simulation.commands.SimulationWithMonitorRunner$SimulationRunnerThread.run(SimulationWithMonitorRunner.java:93) Caused by: java.lang.NullPointerException at org.bonitasoft.simulation.reporting.CSVSimReportStorage.createActivityInstanceFromString(CSVSimReportStorage.java:301) at org.bonitasoft.simulation.reporting.CSVSimReportStorage.createProcessInstanceFromString(CSVSimReportStorage.java:251) at org.bonitasoft.simulation.reporting.CSVSimReportStorage.access$000(CSVSimReportStorage.java:57) at org.bonitasoft.simulation.reporting.CSVSimReportStorage$1.processDataLine(CSVSimReportStorage.java:124) at net.sf.csv4j.CSVStreamProcessor.processStream(CSVStreamProcessor.java:149) ... 5 more

What could be the problem?