diff --git a/manifests/plugin.pp b/manifests/plugin.pp
index 4139ce0..658ac2a 100644
--- a/manifests/plugin.pp
+++ b/manifests/plugin.pp
@@ -1,87 +1,89 @@
 # munin::plugin
 #
 # Parameters:
 #
-# - ensure: link, present, absent
+# - ensure: "link", "present", "absent" or "". Default is "". The
+#   ensure parameter is mandatory for installing a plugin.
 # - source: when ensure => present, source file
 # - target: when ensure => link, link target
 # - config: array of lines for munin plugin config
 # - config_label: label for munin plugin config
 
 define munin::plugin (
-    $ensure=undef,
+    $ensure='',
     $source=undef,
     $target=undef,
     $config=undef,
     $config_label=undef,
 )
 {
 
     include munin::node
 
     $plugin_share_dir=$munin::node::plugin_share_dir
     validate_absolute_path($plugin_share_dir)
 
     File {
         require => Package[$munin::node::package_name],
         notify  => Service[$munin::node::service_name],
     }
 
+    validate_re($ensure, '^(|link|present|absent)$')
     case $ensure {
-        present: {
+        'present': {
             $handle_plugin = true
-            $plugin_ensure = present
+            $plugin_ensure = 'present'
         }
-        absent: {
+        'absent': {
             $handle_plugin = true
-            $plugin_ensure = absent
+            $plugin_ensure = 'absent'
         }
-        link: {
+        'link': {
             $handle_plugin = true
-            $plugin_ensure = link
+            $plugin_ensure = 'link'
             case $target {
-                '': {
+                undef: {
                     $plugin_target = "${munin::node::plugin_share_dir}/${title}"
                 }
                 /^\//: {
                     $plugin_target = $target
                 }
                 default: {
                     $plugin_target = "${munin::node::plugin_share_dir}/${target}"
                 }
             }
         }
         default: {
             $handle_plugin = false
         }
     }
 
     if $config {
         $config_ensure = $ensure ? {
             absent  => absent,
             default => present,
         }
     }
     else {
         $config_ensure = absent
     }
 
 
     if $handle_plugin {
         # Install the plugin
         file {"${munin::node::config_root}/plugins/${name}":
             ensure => $plugin_ensure,
             source => $source,
             target => $plugin_target,
             mode   => '0755',
         }
     }
 
     # Config
 
     file{ "${munin::node::config_root}/plugin-conf.d/${name}.conf":
       ensure  => $config_ensure,
       content => template('munin/plugin_conf.erb'),
     }
 
 }