How to select Value of a combobox on system form?

Dear friends,

My requirement is to select the value of transaction category combobox which is on Purchase order form automatically through my program.

But when i do this

oformPO.PaneLevel = oformPO.Items.Item("2068").ToPane

Dim ocombo1 As SAPbouiCOM.ComboBox

ocombo1 = oformPO.Items.Item("2068").Specific

ocombo1.Select("T2", SAPbouiCOM.BoSearchKey.psk_ByDescription)

It gives this error "Argument is out of Range [66000-91]"

although the Description is present in Valid values collection of combo I also tried by using SAPbouiCOM.BoSearchKey.psk_Index

but it gives the same error

Note: oformPO is set to newly opened Purchase Order form in Add mode

Regards,

Atul.