TaskCannotModifyProjectSummary Error. But not updating the Project Summary Task

Hi,

I have been trying to update few Task level custom fields for non-summary tasks using PSI. It was working for few project plans and very few plans throwing "TaskCannotModifyProjectSummary" error. In code, clearly ignoring the Project& Task summary task.

 foreach (ProjectDataSet.TaskRow tskRow in projectDs.Task)
            {
                if (tskRow.TASK_ID != 0 &&  tskRow.IsTASK_IS_SUMMARYNull() == false && tskRow.TASK_IS_SUMMARY == false)
                {
                    if (statusManagerUid.Equals(Guid.Empty) == false)
                        tskRow.StatusManager = statusManagerUid;
                }
            }

With curiosity, i tried to update the project without updating any custom field or any other entities. Just Read project dataset-->Checkout-->UpdateProject--> Publish-->Checkin(following is the code for the same). But still, i was getting the same "TaskCannotModifyProjectSummary". So is this something wrong with the schedule or any additional settings need to do before updating project through PSI?

                ProjectDataSet projectDs = new ProjectDataSet();
                projectDs = projectSvc.ReadProject(sm.ProjectUID, DataStoreEnum.WorkingStore);
                projectSvc.CheckOutProject(ProjectUID, sessionUid, "Updating CF Through Tool");

                Guid jobUid = Guid.NewGuid();
                // Update
                projectSvc.QueueUpdateProject(jobUid, sessionUid, projectDs, false);
                WaitForQueue(q, jobUid);

                // Publish
                jobUid = Guid.NewGuid();
                projectSvc.QueuePublish(jobUid, sm.ProjectUID, true, string.Empty);
                WaitForQueue(q, jobUid);

                jobUid = Guid.NewGuid();

                // Checkin
                projectSvc.QueueCheckInProject(jobUid, sm.ProjectUID, false, sessionUid, "Updating CF. Publising through Tool");
                WaitForQueue(q, jobUid);

I'm happy to provide, if any further details are required to fix this problem.

Thanks, Ram