diff --git a/lib/worldpay_cnp/xml/nokogiri.rb b/lib/worldpay_cnp/xml/nokogiri.rb
index ee36f36..354ff7c 100644
--- a/lib/worldpay_cnp/xml/nokogiri.rb
+++ b/lib/worldpay_cnp/xml/nokogiri.rb
@@ -26,16 +26,16 @@ class Serializer
         include XML::Serializer
 
         def call(hash)
-          ::Nokogiri::XML::Document.new.tap { |d| add_xml_elements!(d, hash) }.to_s
+          ::Nokogiri::XML::Document.new.tap { |d| add_xml_elements!(d, nil, hash) }.to_s
         end
 
         private
 
-        def attributes_or_elements!(parent, key, value)
+        def attributes_or_elements!(document, parent, key, value)
           return parent[attribute_name(key)] = text_with(value) if attribute?(key)
-          element = ::Nokogiri::XML::Element.new(key.to_s, parent)
+          element = ::Nokogiri::XML::Element.new(key.to_s, document)
           parent.add_child(element)
-          add_xml_elements!(element, value)
+          add_xml_elements!(document, element, value)
         end
 
         def insert_text!(element, text)
diff --git a/lib/worldpay_cnp/xml/serializer.rb b/lib/worldpay_cnp/xml/serializer.rb
index a262cff..87a99ef 100644
--- a/lib/worldpay_cnp/xml/serializer.rb
+++ b/lib/worldpay_cnp/xml/serializer.rb
@@ -7,7 +7,7 @@ def call(hash)
 
       private
 
-      def attributes_or_elements!(parent, key, value)
+      def attributes_or_elements!(document, parent, key, value)
         raise NotImplementedError
       end
 
@@ -15,12 +15,13 @@ def insert_text!(element, text)
         raise NotImplementedError
       end
 
-      def add_xml_elements!(parent, obj)
+      def add_xml_elements!(document, parent, obj)
+        parent = document if parent.nil?
         case obj
         when Hash
-          obj.each { |key, value| attributes_or_elements!(parent, key, value) }
+          obj.each { |key, value| attributes_or_elements!(document, parent, key, value) }
         when Array
-          obj.each { |value| add_xml_elements!(parent, value) }
+          obj.each { |value| add_xml_elements!(document, parent, value) }
         else
           insert_text!(parent, obj)
         end