diff --git a/core/lib/Drupal/Core/Plugin/DefaultSinglePluginBag.php b/core/lib/Drupal/Core/Plugin/DefaultSinglePluginBag.php
index 13428ae..a9618c0 100644
--- a/core/lib/Drupal/Core/Plugin/DefaultSinglePluginBag.php
+++ b/core/lib/Drupal/Core/Plugin/DefaultSinglePluginBag.php
@@ -12,6 +12,12 @@
 
 /**
  * Provides a default plugin bag for a plugin type.
+ *
+ * A plugin bag usually stores multiple plugins, and is used to lazily
+ * instantiate them. When only one plugin is needed, it is still best practice
+ * to encapsulate all of the instantiation logic in a plugin bag. This class can
+ * be used directly, or subclassed to add further exception handling in
+ * self::initializePlugin().
  */
 class DefaultSinglePluginBag extends PluginBag {
 
@@ -23,6 +29,8 @@ class DefaultSinglePluginBag extends PluginBag {
   protected $manager;
 
   /**
+   * An array of configuration to instantiate the plugin with.
+   *
    * @var array
    */
   protected $configuration;
diff --git a/core/modules/block/lib/Drupal/block/BlockPluginBag.php b/core/modules/block/lib/Drupal/block/BlockPluginBag.php
index e71fcca..6a2fd12 100644
--- a/core/modules/block/lib/Drupal/block/BlockPluginBag.php
+++ b/core/modules/block/lib/Drupal/block/BlockPluginBag.php
@@ -9,6 +9,7 @@
 
 use Drupal\Component\Plugin\Exception\PluginException;
 use Drupal\Component\Plugin\PluginManagerInterface;
+use Drupal\Component\Utility\String;
 use Drupal\Core\Plugin\DefaultSinglePluginBag;
 
 /**
@@ -37,7 +38,7 @@ public function __construct(PluginManagerInterface $manager, array $instance_ids
    */
   protected function initializePlugin($instance_id) {
     if (!$instance_id) {
-      throw new PluginException(format_string("The block '@block' did not specify a plugin.", array('@block' => $this->blockId)));
+      throw new PluginException(String::format("The block '@block' did not specify a plugin.", array('@block' => $this->blockId)));
     }
 
     try {