How to use UpdateItem() in DataServiceTransaction

Hello

I am using the following code to push an entry to the client and also create a new one in the database. However subsequently I want to just update the previous entry with new volume and price. So I use the updateItem() as shown below.  Please note that the entry in the database is already present, I am just updating it as part of this test..

        DataServiceTransaction dtx = DataServiceTransaction.begin(false); 

        dtx.setSendChangesToAdapter(true);

        Positions p1 = new Positions();

        //p1.setPositionsId(new Integer(1));

        p1.setPositionsSymbol(new String(symbol));

        p1.setPositionsVolume(new Double(volume));

        p1.setPositionsPrice(new Double(price));

        p1.setPositionsServerTimestamp(new Date()); 

        //dtx.createItem("WorkStation.Positions", p1); 

        dtx.updateItem("WorkStation.Positions", p1, null, null); 

        dtx.commit();

It is however throwing an exception flex.messaging.MessageException at

flex.data.DataService.sendBatchToAdapter(DataService.java 1630)

...

...

Any ideas whats going wrong here...

thanks

RK