ActivityInstanceDestination.User.FQN doesn't work with Direct Action

Hi

I want to get the user that finished an activity in a server event.

In my activity i've a client event and then a server event which saves the user that finished the client event to a process field (name="Arbitre") : 

           K2.ProcessInstance.DataFields["Arbitre"].Value = K2.ActivityInstanceDestination.User.FQN;

I deploy my process and start a new instance. I executed the client event without loading form. I use workspace and in the contextual menu of my task i select Action and Accept (name of my action).

Then my process field's value is "K2Server". It is the name of the K2 host server and not user's name !!

Why ?

Thank you for your replies