Heed the warning about Optional's default being non-undef.

Having another default led to weird errors.

manifests/config.pp

@@ -93,7 +93,7 @@ define mpd::config (
   String $pid_file                                            = "${mpd_home}/pid",
   String $playlist_directory                                  = "${music_directory}/playlists",
   String $state_file                                          = "${mpd_home}/state",
-  Optional[String] $user                                      = 'mpd',
+  Optional[String] $user                                      = undef,
   String $bind_to_address                                     = 'any',
   Integer $port                                               = 6600,
   Enum['default','secure','verbose'] $log_level               = 'default',