delegate sharepoint task programatically

Hi, 

I have a SharePoint Workflow task created and I added task delegation like this.

SPListItem listItem
                = currentWeb.Lists.TryGetList(taskListName).GetItemById(taskId);
            listItem["AssignedTo"] = delegateTouser;
            listItem.Update();

once the delegated user approve or reject the task, it gives following error. 

"Error in request approval action. Unauthorized attempt to update approval task by WIN-4IVE6RU7V84\testuser1. The response has not been recorded."

How to fix this?