diff --git a/lib/pg_ldap_sync/compat.rb b/lib/pg_ldap_sync/compat.rb
index 24f7845..8144a43 100644
--- a/lib/pg_ldap_sync/compat.rb
+++ b/lib/pg_ldap_sync/compat.rb
@@ -1,6 +1,7 @@
 #!/usr/bin/env ruby
 
 class Hash
+  # transform_keys was added in ruby-2.5
   def transform_keys
     map do |k, v|
       [yield(k), v]