libxml-ruby and multiple namespaces

Question

having this xml declaration:

<didl:DIDL xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:didl="urn:mpeg:mpeg21:2002:02-DIDL-NS"
        xsi:schemaLocation="urn:mpeg:mpeg21:2002:02-DIDL-NS      http://standards.iso.org/ittf/PubliclyAvailableStandards/MPEG-21_schema_files/did/didmodel.xsd"&gt;

what is the way to define two namespaces (didl, xsi) in libxml-ruby?

i do:

doc.find('/didl:DIDL', ['didl:urn:mpeg:mpeg21:2002:02-DIDL-NS'])

the xpath works fine, but i got the annoying output:

Error: Namespace prefix xsi for schemaLocation on DIDL is not defined at ....

thank you