diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index acb23b71..c7664258 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -337,9 +337,8 @@ module ActiveModel
       return Enumerator.new unless object
 
       Enumerator.new do |y|
-        self.class._reflections.values.each do |reflection|
+        self.class._reflections.each do |key, reflection|
           next if reflection.excluded?(self)
-          key = reflection.options.fetch(:key, reflection.name)
           next unless include_directive.key?(key)
 
           y.yield reflection.build_association(self, instance_options, include_slice)