The method 'Skip' is only supported for sorted input in LINQ to Entities

What could be causing this problem?

public ActionResult Index(int page = 0)
{
    const int pageSize = 3;
    var areas = repo.FindAllAreas();
    var paginatedArea = new PaginatedList<Area>(areas, page, pageSize);

    return View(paginatedArea);
}


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace UTEPSA.Controllers
{
    class PaginatedList<T> : List<T>
    {
        public int PageIndex { get; private set; }
        public int PageSize { get; private set; }
        public int TotalCount { get; private set; }
        public int TotalPages { get; private set; }
        public PaginatedList(IQueryable<T> source, int pageIndex, int pageSize)
        {
            PageIndex = pageIndex;
            PageSize = pageSize;
            TotalCount = source.Count();
            TotalPages = (int)Math.Ceiling(TotalCount / (double)PageSize);
//ERROR HERE->>this.AddRange(source.Skip(PageIndex * PageSize).Take(PageSize));
        }
        public bool HasPreviousPage
        {
            get
            {
                return (PageIndex > 0);
            }
        }
        public bool HasNextPage
        {
            get
            {
                return (PageIndex + 1 < TotalPages);
            }
        }
    }
}

Any suggestions?