why is this wrong

not_right.rb

require "nokogiri"

xml = Nokogiri::XML(File.open("some_shit.xml"))

xml.at_css("xmldata")
# => nil

xml.xpath("//soap12:Envelope")
# => works, returns element and children

xml.xpath("//soap12:Envelope//soap12:Body")
# => works, returns element and children

xml.xpath("//soap12:Envelope//soap12:Body//postdata")
# => suddenly does not work, won't detect the <postdata> element

some_shit.xml

<?xml version="1.0" encoding="utf-8" ?>
<soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
  <soap12:Header>
    <cAuthentication xmlns="http://ar.masstech-pts.org/">
      <UserName>username</UserName>
      <Password>password1</Password>
    </cAuthentication>
  </soap12:Header>
  <soap12:Body>
    <postdata xmlns="http://ar.masstech-pts.org/">
      <xmldata></xmldata>
    </postdata>
  </soap12:Body>
</soap12:Envelope>

the original query is correct, im saying the element 'postdata' in the XML is defining its own namespace which is putting it out of reach of the normal query. i dont understand xml namespaces that well but if postdata has to have a namespace, perhaps use a namespace prefix, such as how soap12 works.