diff --git a/lib/active_model/serializer/associations.rb b/lib/active_model/serializer/associations.rb
index 2ee98f65..30ca5f4f 100644
--- a/lib/active_model/serializer/associations.rb
+++ b/lib/active_model/serializer/associations.rb
@@ -177,8 +177,10 @@ module ActiveModel
         end
 
         def serialize_ids
-          if associated_object
-            id = associated_object.read_attribute_for_serialization(embed_key)
+          object = associated_object
+
+          if object
+            id = object.read_attribute_for_serialization(embed_key)
             if polymorphic?
               {
                 :type => polymorphic_key,