diff --git a/test/fixtures/poro.rb b/test/fixtures/poro.rb
index 28ee08d5..573b3fa8 100644
--- a/test/fixtures/poro.rb
+++ b/test/fixtures/poro.rb
@@ -16,6 +16,12 @@ class Model < ActiveModelSerializers::Model
     end
   end
 
+  # required for ActiveModel::AttributeAssignment#_assign_attribute
+  # in Rails 5
+  def respond_to_missing?(method_name, _include_private = false)
+    attributes.key?(method_name.to_s.tr('=', '').to_sym) || super
+  end
+
   def cache_key_with_digest
     "#{cache_key}/#{FILE_DIGEST}"
   end