Passing variant argument in COM?

I'm trying to control VisualDSP++ and have trouble passing simple numbers as Variant to the most important methods

Methods for class Interface.VisualDSP___IDDE.IADspSessionList: handle CreateSession(handle, Variant) handle Item(handle, Variant)

I tried visualDSPhandle.SessionList.Item(1) and it returns "??? Invoke Error, Dispatch Exception: The parameter is incorrect."

Any ideas? Thanks in advance