diff --git a/core/lib/Drupal/Core/Breadcrumb/BreadcrumbManager.php b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbManager.php
index f5af77e..5396e9e 100644
--- a/core/lib/Drupal/Core/Breadcrumb/BreadcrumbManager.php
+++ b/core/lib/Drupal/Core/Breadcrumb/BreadcrumbManager.php
@@ -16,19 +16,18 @@
 class BreadcrumbManager implements BreadcrumbBuilderInterface {
 
   /**
-   * Holds the array of breadcrumb builders, sorted by priority.
+   * Holds arrays of breadcrumb builders, keyed by priority.
    *
    * @var array
-   *   An array of breadcrumb builders sorted by priorities.
    */
   protected $builders = array();
 
   /**
    * Holds the array of breadcrumb builders sorted by priority.
    *
+   * Set to NULL if the array needs to be re-calculated.
+   *
    * @var array|NULL
-   *   An array of breadcrumb builders sorted by priorities, or
-   *   NULL, if the array needs to be re-calculated.
    */
   protected $sortedBuilders;
 
@@ -42,6 +41,8 @@ class BreadcrumbManager implements BreadcrumbBuilderInterface {
    */
   public function addBuilder(BreadcrumbBuilderInterface $builder, $priority) {
     $this->builders[$priority][] = $builder;
+    // Force the builders to be re-sorted.
+    $this->sortedBuilders = NULL;
   }
 
   /**
@@ -49,7 +50,7 @@ public function addBuilder(BreadcrumbBuilderInterface $builder, $priority) {
    */
   public function build(array $attributes) {
     // Call the build method of registered breadcrumb builders,
-    // until one of them returns array.
+    // until one of them returns an array.
     foreach ($this->getSortedBuilders() as $builder) {
       $breadcrumb = $builder->build($attributes);
       if (!isset($breadcrumb)) {
@@ -79,10 +80,10 @@ protected function getSortedBuilders() {
     if (!isset($this->sortedBuilders)) {
       // Sort the builders according to priority.
       krsort($this->builders);
-      // Merge the nested $this->builders array into $this->sortedBuilders.
+      // Merge nested builders from $this->builders into $this->sortedBuilders.
       $this->sortedBuilders = array();
-      foreach ($this->builders as $builder) {
-        $this->sortedBuilders = array_merge($this->sortedBuilders, $builder);
+      foreach ($this->builders as $builders) {
+        $this->sortedBuilders = array_merge($this->sortedBuilders, $builders);
       }
     }
     return $this->sortedBuilders;