nokogiri doc check for avm::xmp

lib/avm/xmp.rb

@@ -24,6 +24,9 @@ module AVM
 
     def initialize(doc = nil)
       @doc = doc || empty_xml_doc
+
+      raise StandardError.new('not a Nokogiri node') if !@doc.kind_of?(::Nokogiri::XML::Node)
+
       ensure_namespaces!
       ensure_descriptions_findable!
     end

spec/avm/xmp_spec.rb

@@ -6,6 +6,14 @@ describe AVM::XMP do
 
   subject { xmp }
 
+  describe '#initialize' do
+    context 'not a nokogiri document' do
+      let(:xmp) { self.class.describes.new("definitely not nokogiri node") }
+
+      it { expect { xmp }.to raise_error(StandardError, /not a Nokogiri node/) }
+    end
+  end
+
   describe '#get_refs' do
     before {
       xmp.get_refs do |refs|