diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php
index 05921ed..bb0a6ee 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockUiTest.php
@@ -28,6 +28,11 @@ class BlockUiTest extends WebTestBase {
    */
   protected $adminUser;
 
+  /**
+   * Stores the current test theme name.
+   */
+  protected $currentTheme;
+
   public static function getInfo() {
     return array(
       'name' => 'Block UI',
@@ -46,17 +51,25 @@ function setUp() {
     $this->drupalLogin($this->adminUser);
 
     // Enable some test blocks.
+    $this->currentTheme = \Drupal::config('system.theme')->get('default');
     $this->testBlocks = array(
       array(
         'label' => 'Tools',
         'tr' => '5',
         'plugin_id' => 'system_menu_block:tools',
         'settings' => array('region' => 'sidebar_second', 'id' => 'tools'),
+        'test_weight' => '-2',
+      ),
+      array(
+        'label' => 'Main page content',
+        'tr' => '8',
+        'plugin_id' => 'system_main_block',
+        'settings' => array('region' => 'content', 'id' => $this->currentTheme . '_content'),
         'test_weight' => '-1',
       ),
       array(
         'label' => 'Powered by Drupal',
-        'tr' => '12',
+        'tr' => '13',
         'plugin_id' => 'system_powered_by_block',
         'settings' => array('region' => 'footer', 'id' => 'powered'),
         'test_weight' => '0',
@@ -104,12 +117,12 @@ function testBlockAdminUiPage() {
     foreach ($this->testBlocks as $values) {
       // Check if the region and weight settings changes have persisted.
       $this->assertOptionSelected(
-        'edit-blocks-' . $values['settings']['id']  . '-region',
+        'edit-blocks-' . str_replace('_', '-', $values['settings']['id'])  . '-region',
         'header',
         'The block "' . $values['label'] . '" has the correct region assignment (header).'
       );
       $this->assertOptionSelected(
-        'edit-blocks-' . $values['settings']['id']  . '-weight',
+        'edit-blocks-' . str_replace('_', '-', $values['settings']['id'])  . '-weight',
         $values['test_weight'],
         'The block "' . $values['label'] . '" has the correct weight assignment (' . $values['test_weight'] . ').'
       );
@@ -117,6 +130,21 @@ function testBlockAdminUiPage() {
   }
 
   /**
+   * Tests that the main page content block needs to be assigned in some region.
+   */
+  public function testSystemMainContentBlockAssigned() {
+    // Try to disable main page content block.
+    $edit = array('blocks[' . $this->currentTheme . '_content][region]' => -1);
+    $this->drupalPostForm('admin/structure/block', $edit, t('Save blocks'));
+    $this->assertText(t('Main page content block needs to be placed in some region.'), 'Failed to disable main page content block.');
+
+    // Try to set main page content block in main content region.
+    $edit = array('blocks[' . $this->currentTheme . '_content][region]' => 'content');
+    $this->drupalPostForm('admin/structure/block', $edit, t('Save blocks'));
+    $this->assertText(t('The block settings have been updated.'), 'Main page content block successfully moved to Content region.');
+  }
+
+  /**
    * Tests the block categories on the listing page.
    */
   public function testCandidateBlockList() {