diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index c24ae4e430..971bdd08b1 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -398,21 +398,19 @@ module ActiveModel
       type = options.delete(:message) if options[:message].is_a?(Symbol)
 
       if @base.class.respond_to?(:i18n_scope)
-        defaults = @base.class.lookup_ancestors.map do |klass|
-          [ :"#{@base.class.i18n_scope}.errors.models.#{klass.model_name.i18n_key}.attributes.#{attribute}.#{type}",
-            :"#{@base.class.i18n_scope}.errors.models.#{klass.model_name.i18n_key}.#{type}" ]
+        i18n_scope = @base.class.i18n_scope.to_s
+        defaults = @base.class.lookup_ancestors.flat_map do |klass|
+          [ :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.attributes.#{attribute}.#{type}",
+            :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.#{type}" ]
         end
+        defaults << :"#{i18n_scope}.errors.messages.#{type}"
       else
         defaults = []
       end
 
-      defaults << :"#{@base.class.i18n_scope}.errors.messages.#{type}" if @base.class.respond_to?(:i18n_scope)
       defaults << :"errors.attributes.#{attribute}.#{type}"
       defaults << :"errors.messages.#{type}"
 
-      defaults.compact!
-      defaults.flatten!
-
       key = defaults.shift
       defaults = options.delete(:message) if options[:message]
       value = (attribute != :base ? @base.send(:read_attribute_for_validation, attribute) : nil)