diff --git a/manifests/config.pp b/manifests/config.pp
index e802c8b..0ff4c31 100644
--- a/manifests/config.pp
+++ b/manifests/config.pp
@@ -1,78 +1,78 @@
 # == Class hitch::config
 #
 # This class is called from hitch for service config.
 #
 # @api private
 class hitch::config (
   Stdlib::Absolutepath $config_root,
   Stdlib::Absolutepath $config_file,
   Stdlib::Absolutepath $dhparams_file,
   Boolean $purge_config_root,
   String $file_owner,
   String $user,
   String $group,
-  Optional[String] $dhparams_content,
   Enum['on','off'] $write_proxy_v2,
   Variant[String, Array] $frontend,
   String $backend,
   String $ciphers,
   Variant[Integer, Enum['auto']] $workers,
   Enum['on','off'] $prefer_server_ciphers,
-  Optional[String] $alpn_protos,
-  Optional[String] $tls_protos,
+  Optional[String] $dhparams_content = undef,
+  Optional[String] $alpn_protos = undef,
+  Optional[String] $tls_protos = undef,
 ) {
 
   case $frontend {
     Array: {
       $frontend_array = $frontend
     }
     String: {
       $frontend_string = $frontend
     }
     default: {
       fail('invalid frontend')
     }
   }
 
   file { $config_root:
     ensure  => directory,
     recurse => true,
     purge   => $purge_config_root,
     owner   => $file_owner,
     group   => $group,
     mode    => '0750',
   }
 
   concat { $config_file:
     ensure => present,
   }
 
   if $dhparams_content {
     file { $dhparams_file:
       ensure  => present,
       owner   => $file_owner,
       group   => $group,
       mode    => '0640',
       content => $dhparams_content,
     }
   }
   else {
     exec { "${title} generate dhparams":
       path    => '/usr/local/bin:/usr/bin:/bin',
       command => "openssl dhparam -out ${dhparams_file} 2048",
       creates => $dhparams_file,
     }
 
     -> file { $dhparams_file:
       ensure => present,
       owner  => $file_owner,
       group  => $group,
       mode   => '0640',
     }
   }
 
   concat::fragment { "${title} config":
     content => template('hitch/hitch.conf.erb'),
     target  => $config_file,
   }
 }