diff --git a/lib/iiif/presentation/annotation.rb b/lib/iiif/presentation/annotation.rb
index 9c98e63..e218316 100644
--- a/lib/iiif/presentation/annotation.rb
+++ b/lib/iiif/presentation/annotation.rb
@@ -7,7 +7,11 @@ class Annotation < AbstractResource
       TYPE = 'oa:Annotation'
 
       def required_keys
-        super + %w{ motivation }
+        super + %w{ motivation on }
+      end
+
+      def string_only_keys
+        super + %w{ on }
       end
 
       def abstract_resource_only_keys