diff --git a/core/modules/block/lib/Drupal/block/Tests/Views/DisplayBlockTest.php b/core/modules/block/lib/Drupal/block/Tests/Views/DisplayBlockTest.php
index 668bd77..5da0049 100644
--- a/core/modules/block/lib/Drupal/block/Tests/Views/DisplayBlockTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/Views/DisplayBlockTest.php
@@ -85,7 +85,7 @@ public function testBlockCategory() {
     $link = $this->xpath('//a[@id="views-block-1-block-category" and normalize-space(text())=:label]', array(':label' => $label));
     $this->assertTrue(!empty($link));
     $this->clickLink($label);
-    $category = $this->randomString();
+    $category = '#' . $this->randomString();
     $this->drupalPostForm(NULL, array('block_category' => $category), t('Apply'));
 
     // Clone the block after changing the category.