diff --git a/lib/rack/session/abstract/id.rb b/lib/rack/session/abstract/id.rb
index 633f27038..1ff0e7ae4 100644
--- a/lib/rack/session/abstract/id.rb
+++ b/lib/rack/session/abstract/id.rb
@@ -28,9 +28,9 @@ def private_id
       end
 
       alias :cookie_value :public_id
+      alias :to_s :public_id
 
       def empty?; false; end
-      def to_s; raise; end
       def inspect; public_id.inspect; end
 
       private