k2 Approval from web aspx

i have function like this

private void k2action(int action)
        {
            try
            {
                oConn = new WF.Connection();
                oConn.Open(ConfigurationManager.AppSettings["K2Server"], string.Format("{0},{1},{2}",
                               ConfigurationManager.AppSettings["domain"], ConfigurationManager.AppSettings["username"],
                              ConfigurationManager.AppSettings["password"]));

                WF.WorklistItem wfItem = oConn.OpenWorklistItem(Request.Params["sn"],"ASP");
                oConn.ImpersonateUser(user);
                wfItem.Actions[action].Execute();
            }
            finally
            {
                if (oConn != null)
                    oConn.Close();
            }
        }

it get an error in wfItem.Actions[action].Execute(); and it say it was null and Object reference not set to an instance of an object.

can anyone help me here??

when i debug it and check the worklist item, the folio and serial number is correct when i check it but the action count is 0

oyeah i can approve it via k2 workspace though

thx