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

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

I have simple web app with gridview.  data is loaded through entity 
SelectMethod="GetDepartments"
            ItemType="Model_Binding.Dpt"

        private SchoolEntities db = new SchoolEntities();
        public IQueryable<Dpt> GetDepartments()
        {
            var query = this.db.Dpts
              .Include(c => c.Users);
            return query;
        }
 
Of course I allow sorting and paging - therefore that error.
To fix it add DataKeyNames

        <asp:GridView ID="GridView1" runat="server" SelectMethod="GetDepartments"
            ItemType="Model_Binding.Dpt" AllowPaging="True" AllowSorting="True"
            AutoGenerateColumns="False" DataKeyNames="DepartmentId" PageSize="1">