diff --git a/manifests/params/validate.pp b/manifests/params/validate.pp
index 9578b8e..f2e6bb1 100644
--- a/manifests/params/validate.pp
+++ b/manifests/params/validate.pp
@@ -37,17 +37,17 @@
   $is_director           = '',
   $is_storage            = '',
   $logwatch_enabled      = '',
-  $mail_to               = '',
-  $mail_to_daemon        = '',
-  $mail_to_on_error      = '',
-  $mail_to_operator      = '',
+  $mail_to               = undef,
+  $mail_to_daemon        = undef,
+  $mail_to_on_error      = undef,
+  $mail_to_operator      = undef,
   $manage_bat            = '',
   $manage_config_dir     = '',
   $manage_console        = '',
   $manage_db             = '',
   $manage_db_tables      = '',
   $manage_logwatch       = '',
-  $plugin_dir            = '',
+  $plugin_dir            = undef,
   $storage_default_mount = '',
   $storage_server        = '',
   $tls_allowed_cn        = '',