diff --git a/lib/mongoid/slug.rb b/lib/mongoid/slug.rb
index 1be4199..454c03f 100644
--- a/lib/mongoid/slug.rb
+++ b/lib/mongoid/slug.rb
@@ -250,9 +250,7 @@ def to_param
 
     # @return [String] the slug, or nil if the document does not have a slug.
     def slug
-      return _slugs.last if _slugs
-
-      _id.to_s
+      _slugs.last if _slugs
     end
 
     def slug_builder