fake an event call

I have the problem to fake a call to an event. I use NUnit as well. Here is the code:

[Test, Isolated]
public void TestEventCall
{
    ... some definitions

     string resultingServer, resultingItem;
     int resultingValue;

    // here I set up the listener
    opc.OnEventFired += (server, itemName, value) =>
                                    {
                                        resultingServer = server;
                                        resultingItem = itemName;
                                        resultingValue = value;
                                    }

  Isolate.Invoke.Event (() => opc.OnEventFired += null, serverName, itemName, 200);

  Assert.AreEqual (resultingServer, serverName);
  Assert.AreEqual (resultingItem, itemName);
  Assert.AreEqual (resultingValue, 200);
}

The issue is it does not compile. The error states that the event is not supported and I shall use the accessor methode 'add_OnEventFired'.
The event is however public at the class to be tested.

Can anyone help me here?