diff --git a/lib/oga/xml/attribute.rb b/lib/oga/xml/attribute.rb
index 7b00ad1..cbe2821 100644
--- a/lib/oga/xml/attribute.rb
+++ b/lib/oga/xml/attribute.rb
@@ -82,7 +82,7 @@ module Oga
       #
       def to_xml
         if namespace_name
-          full_name = "#{namespace.name}:#{name}"
+          full_name = "#{namespace_name}:#{name}"
         else
           full_name = name
         end
diff --git a/spec/oga/xml/attribute_spec.rb b/spec/oga/xml/attribute_spec.rb
index 737b7f2..23e1e4f 100644
--- a/spec/oga/xml/attribute_spec.rb
+++ b/spec/oga/xml/attribute_spec.rb
@@ -68,6 +68,16 @@ describe Oga::XML::Attribute do
 
       attr.to_xml.should == 'foo:class="10"'
     end
+
+    example 'include the "xmlns" namespace when present but not registered' do
+      attr = described_class.new(
+        :name           => 'class',
+        :namespace_name => 'xmlns',
+        :element        => Oga::XML::Element.new
+      )
+
+      attr.to_xml.should == 'xmlns:class=""'
+    end
   end
 
   context '#inspect' do