diff --git a/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
index 3ce6900..1951d57 100644
--- a/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -251,15 +251,18 @@ public function hasClass($class) {
    * Implements the magic __toString() method.
    */
   public function __toString() {
-    $return = array();
+    $return = '';
     /** @var \Drupal\Core\Template\AttributeValueBase $value */
     foreach ($this->storage as $name => $value) {
-      $return[] = $value->render();
+      $rendered = $value->render();
+      if ($rendered) {
+        $return .= ' ' . $rendered;
+      }
     }
     // The implementations of AttributeValueBase::render() call
     // htmlspecialchars() on the attribute name and value so we are confident
     // that the return value can be set as safe.
-    return SafeMarkup::set(implode(' ', $return));
+    return SafeMarkup::set($return);
   }
 
   /**