Object doesn't support this property or method: Field.AsString

I attempted to run to the sub procedure below in TestComplete but encountered the exception:

Object doesn't support this property or method: Field.AsString



What could be the issue?

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Sub Unit1



          dim dd, field, combo

          set combo = Sys.Process("Exnet").VCLObject("AccountForm").VCLObject("PageControl1").VCLObject("DetailsPage").VCLObject("GroupBox4").VCLObject("SalespersonCombo")         

          set dd = combo.ListSource.Dataset

          msgbox dd.RecordCount

          set field = dd.FieldByName("STAFFNO")

         

          dd.DisableControls

          dd.First

          msgbox field.AsString

          dd.EnableControls



End Sub

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combo is a TDBLookupComboBox