diff --git a/lib/mongoid/slug.rb b/lib/mongoid/slug.rb
index <HASH>..<HASH> 100644
--- a/lib/mongoid/slug.rb
+++ b/lib/mongoid/slug.rb
@@ -232,7 +232,8 @@ module Mongoid
           existing_slugs += existing_history_slugs
         end
 
-        # IF OBJECTID, ADD TO EXISTING SLUGS
+        # Do not allow BSON::ObjectIds as slugs
+        existing_slugs << slug if BSON::ObjectId.legal?(slug)
 
         if reserved_words_in_slug.any? { |word| word === slug }
           existing_slugs << slug
Do not allow BSON::ObjectIds as slugs.