DirectoryServices Error: Server doesn't support requested critical extension

I am querying Active Directory to select a list of displaynames and emails. I want to sort the list by displayname, but I am getting the following error:
Server doesn't support requested critical extension
here is my code

VB Code:

DirSearch.Filter = (ConfigurationSettings.AppSettings("AllStaff"))
                DirSearch.PropertiesToLoad.Add("member")
                DirSearch.Sort.PropertyName = "member"
                DirSearch.Sort.Direction = SortDirection.Ascending
                [COLOR=darkred]Result = DirSearch.FindOne[/color]
                For X = 0 To Result.Properties("member").Count - 1
                    UserSearch.Filter = ("(" & Left(Result.Properties("member").Item(X), InStr(Result.Properties("member").Item(X), ",") - 1) & ")")
                    UserSearch.PropertiesToLoad.Add("Mail")
                    UserSearch.PropertiesToLoad.Add("DisplayName")
                    UResult = UserSearch.FindOne()
                    Try
                        Row = Tbl.NewRow
                        Row.Item(0) = UResult.Properties("Mail").Item(0)
                        Row.Item(1) = UResult.Properties("DisplayName").Item(0)
                        Tbl.Rows.Add(Row)
                    Catch ExNull As System.NullReferenceException
                    Catch
                        Response.Redirect("errorpage.aspx?err=" & Err.Number)
                    End Try
                Next X

The line in dark red is where it errors out. Any ideas why this is happening?
if I remove DirSearch.Sort.PropertyName = "member" from the code it will not error out, but the results will not be sorted.

Ultimately what I need is an alphabetical list of Display Names and Emails.

Thanks for your assistance

I found a work around, while it's not the most elegant, it works.
I load the member attribute into an ArrayList object and then sort the list.

VB Code:

DirSearch.Filter = (ConfigurationSettings.AppSettings("AllStaff"))
                DirSearch.PropertiesToLoad.Add("member")
                Result = DirSearch.FindOne
                For X = 0 To Result.Properties("member").Count - 1
                    Member.Add(Left(Result.Properties("member").Item(X), InStr(Result.Properties("member").Item(X), ",") - 1))
                Next
                Member.Sort()
                For X = 0 To Member.Count - 1
                    UserSearch.Filter = ("(" & Member.Item(X) & ")")
                    UserSearch.PropertiesToLoad.Add("Mail")
                    UserSearch.PropertiesToLoad.Add("DisplayName")
                    UResult = UserSearch.FindOne()
                    Try
                        Row = Tbl.NewRow
                        Row.Item(0) = UResult.Properties("Mail").Item(0)
                        Row.Item(1) = UResult.Properties("DisplayName").Item(0)
                        Tbl.Rows.Add(Row)
                    Catch ExNull As System.NullReferenceException
                        'do nothing
                    Catch
                        Response.Redirect("errorpage.aspx?err=" & Err.Number)
                    End Try
                Next