diff --git a/src/Utility/Attributes.php b/src/Utility/Attributes.php
index b6032989c5db664fabd1f34b303709f9bc004fd2..6b1e35814e6c8bc12b6d1230d8183cb47f15235c 100644
--- a/src/Utility/Attributes.php
+++ b/src/Utility/Attributes.php
@@ -2,6 +2,8 @@
 
 namespace Drupal\bootstrap\Utility;
 
+use Drupal\Core\Template\AttributeValueBase;
+
 /**
  * Class to help modify attributes.
  *
@@ -25,6 +27,11 @@ class Attributes extends ArrayObject {
    * @see \Drupal\bootstrap\Utility\Attributes::getClasses()
    */
   public function addClass($class) {
+    // Handle core Attribute based object values.
+    // @see https://www.drupal.org/project/bootstrap/issues/3020266
+    if ($class instanceof AttributeValueBase) {
+      $class = $class->value();
+    }
     $classes = &$this->getClasses();
     $classes = array_unique(array_merge($classes, (array) $class));
   }