ZK web flow - Exception while starting / stopping Tomcat

INFO: Stopping service Catalina
12/11/2009 17:56:29 org.apache.catalina.session.StandardSession writeObject
WARNING: Cannot serialize session attribute zkoss.zwf.FLOW_CONTEXT for session 0215B11358FD7D930ECEB66406435C2D
java.io.NotSerializableException: org.zkoss.zwf.impl.FlowImpl
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
	at java.util.HashMap.writeObject(HashMap.java:1001)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:945)
	at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1461)
	at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1392)
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1150)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
	at org.apache.catalina.session.StandardSession.writeObject(StandardSession.java:1492)
	at org.apache.catalina.session.StandardSession.writeObjectData(StandardSession.java:957)
	at org.apache.catalina.session.StandardManager.doUnload(StandardManager.java:517)
	at org.apache.catalina.session.StandardManager.unload(StandardManager.java:463)
	at org.apache.catalina.session.StandardManager.stop(StandardManager.java:667)
	at org.apache.catalina.core.StandardContext.stop(StandardContext.java:4362)
	at org.apache.catalina.core.ContainerBase.stop(ContainerBase.java:1067)
	at org.apache.catalina.core.ContainerBase.stop(ContainerBase.java:1067)
	at org.apache.catalina.core.StandardEngine.stop(StandardEngine.java:448)
	at org.apache.catalina.core.StandardService.stop(StandardService.java:510)
	at org.apache.catalina.core.StandardServer.stop(StandardServer.java:734)
	at org.apache.catalina.startup.Catalina.stop(Catalina.java:602)
	at org.apache.catalina.startup.Catalina.start(Catalina.java:577)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:295)
	at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:433)