How to add attribute to Nokogiri node?

I'm trying to add an attribute to an existing Nokogiri node. What I've done is this:

node.attributes['foobar'] = Nokogiri::XML::Attr.new('foo', 'bar')

But I get the error:

TypeError Exception: wrong argument type String (expected Data)

What is a Data data type, and how do I add an attribute to the Nokogiri object?

Thanks!