Why does this XmlMarkup return blank?

Rails
1

Both of these methods are in the controller. The method is called
with /.xml;directory. The route completes, I get

XML Parsing Error: no element found
Location: .xml;mytest
Line Number 1, Column 1:

def mytest

respond_to do |format|
  format.html # index.rhtml
  format.xml  { render :xml => try_xml_builder }
end

end

def try_xml_builder

xml = Builder::XmlMarkup.new(:indent=>2)
xml.instruct!
xml.person { |b| b.name("Jim"); b.phone("555-1234") }
return xml

end

If I override to_xml with the same builder script in the mode,
everything is great. There’s something wrong with the xml return value
and render :xml, but I’m not sure what it is. The example for builder
is from the builder pages, and returns the correct xml.

I’m running rails 1.2.2.