rails builder xml: I'm changing namespace but the injected value is removed?

Question

So I have an xml doc being generated based on product info in rails (using the builder markup (http://builder.rubyforge.org/classes/Builder/XmlMarkup.html), but when I use builders namespace syntax, the value I want to print is absent. Here's the code:

xml.tag!("rrs", {"xmlns:g"=>"http://base.google.com/ns/1.0"}) do
 @products.each do |p|
   xml.item do
     xml.id p.id                  # This works and prints: <id>123</id>
     xml.tag!("g:id") { p.id }    # This DOESN'T work and prints: <g:id></g:id>
    end
  end
end 

Notice that the p.id isn't printed when I use the .tag! as the docs say to.

Is this a simple syntax issue I'm screwing up?

Thanks in advance!