ASP.NET MVC PagedList.Mvc error - The method 'OrderBy' must be called before the method 'Skip'.'

I am using ASP.NET MVC 5 with entity framework version 6. I have the PagedList.MVC nuget package installed. I get the error listed below when attempting to sort in my Index view. I'm sure it is something really simple that I am overlooking.

The method 'Skip' is only supported for sorted input in LINQ to Entities. The method 'OrderBy' must be called before the method 'Skip'.'

Controller Code listed below:

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Entity;
using System.Linq;
using System.Net;
using System.Web;
using System.Web.Mvc;
using ITRA.Models;
using PagedList;
using PagedList.Mvc;

namespace ITRA.Controllers
{
    public class SSP_ControlController : Controller
    {
        private RiskAssessmentEntities2 db = new RiskAssessmentEntities2();

        // GET: SSP_Control
        public ViewResult Index(string sortOrder, string currentFilter, string searchString, int? page)
        {
            ViewBag.CurrentSort = sortOrder;
            ViewBag.NISTIDSortParm = String.IsNullOrEmpty(sortOrder) ? "NISTID_asc" : "";
            ViewBag.TitleSortParm = String.IsNullOrEmpty(sortOrder) ? "Title_asc" : "";
            ViewBag.WorksheetStatementSortParm = String.IsNullOrEmpty(sortOrder) ? "WorksheetStatement_asc" : "";

            if (searchString != null)
            {
                page = 1;
            }
            else
            {
                searchString = currentFilter;
            }
            ViewBag.CurrentFilter = searchString;

            var NISTID = from NID in db.SSP_Control
                      select NID;
            var Title = from t in db.SSP_Control
                      select t;
            var WorksheetStatement = from ws in db.SSP_Control
                        select ws;
            if (!String.IsNullOrEmpty(searchString))
            {
                NISTID = NISTID.Where(NID => NID.NISTID.Contains(searchString));               
            }
            switch (sortOrder)
            {
                case "NISTID_asc":
                    NISTID = NISTID.OrderBy(NID => NID.NISTID);
                    break;
                case "Title_asc":
                    Title = Title.OrderBy(t => t.Title);
                    break;
                case "WorksheetStatement_asc":
                    WorksheetStatement = WorksheetStatement.OrderBy(ws => ws.WorksheetStatement);
                    break;
                default:
                    NISTID = NISTID.OrderBy(NID => NID.NISTID);
                    break;
            }
            int pageSize = 10;
                int pageNumber = (page ?? 1);
            return View(NISTID.ToPagedList(pageNumber, pageSize));

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Index View code listed below:

@model PagedList.IPagedList<ITRA.Models.SSP_Control>
@using PagedList.Mvc;
@using PagedList;
<link href="~/Content/PagedList.css" rel="stylesheet" type="text/css" />

@{
    ViewBag.Title = "SSP Controls";
}

<h2>SSP Controls</h2>

<p>
    @Html.ActionLink("Create New", "Create")
</p>

@using (Html.BeginForm("Index", "SSP_Control", FormMethod.Get))
{
    <p>
        Search: @Html.TextBox("SearchString", ViewBag.CurrentFilter as string)
        <input type="submit" value="Search" />
    </p>
}
<table class="table">
    <tr>
        <th>
            @Html.ActionLink("NIST ID", "Index", new { sortOrder = ViewBag.NISTIDSortParm, currentFilter = ViewBag.CurrentFilter })
        </th>
        <th>
            @Html.ActionLink("Title", "Index", new { sortOrder = ViewBag.TitleSortParm, currentFilter = ViewBag.CurrentFilter })
        </th>
        <th>
            @Html.ActionLink("Worksheet Statement", "Index", new { sortOrder = ViewBag.WorksheetStatementSortParm, currentFilter = ViewBag.CurrentFilter })
        </th>
        <th>
            @Html.ActionLink("Control Requirement", "Index")
        </th>
    </tr>

    @foreach (var item in Model)
    {
        <tr>
            <td>
                @Html.DisplayFor(modelItem => item.NISTID)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Title)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.WorksheetStatement)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.ControlRequirement)
            </td>
            <td>
                @Html.ActionLink("Edit", "Edit", new { id = item.ID }) |
                @Html.ActionLink("Details", "Details", new { id = item.ID }) |
                @Html.ActionLink("Delete", "Delete", new { id = item.ID })
            </td>
        </tr>
    }
</table>
<br />
Page @(Model.PageCount < Model.PageNumber ? 0 : Model.PageNumber) of @Model.PageCount
@Html.PagedListPager(Model, page => Url.Action("Index", new { page, sortOrder = ViewBag.CurrentSort, currentFilter = ViewBag.CurrentFilter }))

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