diff -u b/core/modules/picture/lib/Drupal/picture/Tests/PictureThemeFunctionsTest.php b/core/modules/picture/lib/Drupal/picture/Tests/PictureThemeFunctionsTest.php
--- b/core/modules/picture/lib/Drupal/picture/Tests/PictureThemeFunctionsTest.php
+++ b/core/modules/picture/lib/Drupal/picture/Tests/PictureThemeFunctionsTest.php
@@ -57,7 +57,7 @@
     $this->assertTrue(strpos($rendered_element, '<picture>') !== FALSE, 'picture element in theme_picture() correctly renders with a NULL value for the alt option.');
 
     // Test using theme_picture() without passing a value for the alt option.
-    unset($element['alt']);
+    unset($element['#alt']);
     $rendered_element = render($element);
     $expected_result = '<img class="image-style-test" src="' . $url . '" alt="" />';
     $this->assertTrue(strpos($rendered_element, $expected_result) !== FALSE, 'img element in theme_picture() correctly renders the alt option.');