diff --git a/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
index 9d6dd1a4467..693da9a2ae5 100644
--- a/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -534,7 +534,7 @@ class TwigExtension extends \Twig_Extension {
    *
    * @param \Twig_Environment $env
    *   A Twig_Environment instance.
-   * @param mixed[]|\Traversable $value
+   * @param mixed[]|\Traversable|NULL $value
    *   The pieces to join.
    * @param string $glue
    *   The delimiter with which to join the string. Defaults to an empty string.
@@ -545,10 +545,14 @@ class TwigExtension extends \Twig_Extension {
    *   The strings joined together.
    */
   public function safeJoin(\Twig_Environment $env, $value, $glue = '') {
+    if ($value instanceof \Traversable) {
+      $value = iterator_to_array($value, false);
+    }
+
     return implode($glue, array_map(function($item) use ($env) {
       // If $item is not marked safe then it will be escaped.
       return $this->escapeFilter($env, $item, 'html', NULL, TRUE);
-    }, $value));
+    }, (array) $value));
   }
 
 }
diff --git a/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
index 6b9525ea6e8..cb09022c430 100644
--- a/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
+++ b/core/tests/Drupal/Tests/Core/Template/TwigExtensionTest.php
@@ -200,6 +200,20 @@ class TwigExtensionTest extends UnitTestCase {
     ];
     $result = $twig_extension->safeJoin($twig_environment, $items, '<br/>');
     $this->assertEquals('&lt;em&gt;will be escaped&lt;/em&gt;<br/><em>will be markup</em><br/><strong>will be rendered</strong>', $result);
+
+    // Ensure safe_join Twig filter supports Traversable variables.
+    $items = new \ArrayObject([
+      '<em>will be escaped</em>',
+      $markup,
+      ['#markup' => '<strong>will be rendered</strong>'],
+    ]);
+    $result = $twig_extension->safeJoin($twig_environment, $items, ', ');
+    $this->assertEquals('&lt;em&gt;will be escaped&lt;/em&gt;, <em>will be markup</em>, <strong>will be rendered</strong>', $result);
+
+    // Ensure safe_join Twig filter supports empty variables.
+    $items = NULL;
+    $result = $twig_extension->safeJoin($twig_environment, $items, '<br>');
+    $this->assertEmpty($result);
   }
 
   /**