The method 'OrderBy' must be called before the method 'Skip' Exception

I was trying to implement the jQgrid using MvcjQgrid and i got this exception.

System.NotSupportedException was unhandled by user code
  Message=The method 'Skip' is only supported for sorted input in LINQ to Entities. The method 'OrderBy' must be called before the method 'Skip'.

Though OrdeyBy is used before Skip method why it is generating the exception? How can it be solved?

I encountered the exception in the controller:

public ActionResult GridDataBasic(GridSettings gridSettings)
        {          
            var jobdescription = sm.GetJobDescription(gridSettings);
            var totalJobDescription = sm.CountJobDescription(gridSettings);

            var jsonData = new
            {
                total = totalJobDescription / gridSettings.PageSize + 1,
                page = gridSettings.PageIndex,
                records = totalJobDescription,
                rows = (
                    from j in jobdescription
                    select new
                    {
                        id = j.JobDescriptionID,
                        cell = new[] 
                    { 
                        j.JobDescriptionID.ToString(), 
                        j.JobTitle,
                        j.JobType.JobTypeName,
                        j.JobPriority.JobPriorityName,
                        j.JobType.Rate.ToString(),
                        j.CreationDate.ToShortDateString(),
                         j.JobDeadline.ToShortDateString(),

                    }
                    }).ToArray()
            };
            return Json(jsonData, JsonRequestBehavior.AllowGet);
        }

GetJobDescription Method and CountJobDescription Method

public int CountJobDescription(GridSettings gridSettings)
        {
            var jobdescription = _dataContext.JobDescriptions.AsQueryable();

            if (gridSettings.IsSearch)
            {
                jobdescription = gridSettings.Where.rules.Aggregate(jobdescription, FilterJobDescription);
            }
            return jobdescription.Count();
        }

        public IQueryable<JobDescription> GetJobDescription(GridSettings gridSettings)
        {
            var jobdescription = orderJobDescription(_dataContext.JobDescriptions.AsQueryable(), gridSettings.SortColumn, gridSettings.SortOrder);

            if (gridSettings.IsSearch)
            {
                jobdescription = gridSettings.Where.rules.Aggregate(jobdescription, FilterJobDescription);
            }

            return jobdescription.Skip((gridSettings.PageIndex - 1) * gridSettings.PageSize).Take(gridSettings.PageSize);
        }

And Finally FilterJobDescription and OrderJobDescription

private static IQueryable<JobDescription> FilterJobDescription(IQueryable<JobDescription> jobdescriptions, Rule rule)
        {
            if (rule.field == "JobDescriptionID")
            {
                int result;
                if (!int.TryParse(rule.data, out result))
                    return jobdescriptions;
                return jobdescriptions.Where(j => j.JobDescriptionID == Convert.ToInt32(rule.data));

            }

// Similar Statements

            return jobdescriptions;
        }



private IQueryable<JobDescription> orderJobDescription(IQueryable<JobDescription> jobdescriptions, string sortColumn, string sortOrder)
        {
            if (sortColumn == "JobDescriptionID")
                return (sortOrder == "desc") ? jobdescriptions.OrderByDescending(j => j.JobDescriptionID) : jobdescriptions.OrderBy(j => j.JobDescriptionID);

            return jobdescriptions;
        }