Add sorting on a formula in RAS

I'm using VB.Net 2010 with Crystal Reports for VS SP5 (inproc-RAS).

For Each field As CrystalDecisions.ReportAppServer.DataDefModel.Field In MainReport.ReportClientDocument.DataDefController.DataDefinition.FormulaFields    If field.Name = mysortField Then        Dim newRasSortField As New CrystalDecisions.ReportAppServer.DataDefModel.Sort        With newRasSortField           .SortField = field           .Direction = CrSortDirectionEnum.crSortDirectionAscendingOrder        End With        MainReport.ReportClientDocument.DataDefController.SortController.Add(-1, newRasSortField)

The formula exists in the report, but the last line throws an exception "Sorting not possible on this field" (Error -2147213300).

I have no problems adding sort-field from tables.

Any ideas?

BW. where have all your KB-articles gone?

Patrick