Ruby SAPRFC: undefined method pack

Hi

executing the following call

rfc = SAP::Rfc.new(:ashost => "iwdf3018.wdf.sap.corp" , :sysnr => "50" ,:lang => "EN" , :client => "800" , :user => "++" , :passwd => "++" , :trace => "0" )
iface = rfc.discover("BAPI_BUPA_ADDRESSES_GET")
iface.BUSINESSPARTNER.value = "0000002036"
rfc.call(iface)

I am getting the following error:

.../saprfc-0.35-mswin32/lib/SAP/rfc.rb:729:in `discover': undefined method `pack' for #<SAP::Rfc:0x2c8c8d8> (NoMethodError)
 from .../saprfc-0.35-mswin32/lib/SAP/rfc.rb:664:in `each'
 from .../saprfc-0.35-mswin32/lib/SAP/rfc.rb:664:in `discover'

Am I missing a detail? What does it try to tell me? And how can I handle this issue?

Regards,

Stefan