diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index a84b5d6b7..6f1cde5cf 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -24,7 +24,7 @@ module ApplicationHelper
   # Produces a cache key for uniquely identifying cached fragments.
   def cache_key_for(klass, identifier = "all")
     count          = klass.count
-    max_updated_at = klass.maximum(:updated_at).try(:utc).try(:to_s, :number)
+    max_updated_at = klass.maximum(:updated_at).try(:utc).try(:to_fs, :number)
     "#{klass.name.downcase.pluralize}/#{identifier}-#{count}-#{max_updated_at}"
   end