How to approve Flexi Task from sharepont application page

I added a web reference to the Nintex workflow webservice and trying to approve a flexi task i am getting the error message "Server was unable to process request. ---> Item does not exist. It may have been deleted by another user."

Here I took 14 as TaskId and "Workflow Tasks" is the tasklist name. Taskid is there in the task list.

string taskList = web.Lists["Workflow Tasks"].ToString();
NintexWorkflowService.NintexWorkflowWS objWS = new NintexWorkflowService.NintexWorkflowWS();
objWS.Credentials = System.Net.CredentialCache.DefaultCredentials;
objWS.ProcessFlexiTaskResponse("Test", "Approve", 14, taskList);

Can some one help how to approve flexi task from SharePoint application page?