diff --git a/core/includes/common.inc b/core/includes/common.inc
index 91b6c1a..6c54958 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -1296,9 +1296,9 @@ function drupal_strip_dangerous_protocols($uri) {
   static $allowed_protocols;
 
   if (!isset($allowed_protocols)) {
-    // During upgrade path tests system.filter.protocols may be empty. In order
-    // to avoid test breakage, a list of protocols necessary to run tests is
-    // supplied in that case.
+    // filter_xss_admin() is called by the installer and update.php, in which
+    // case the configuration may not exist (yet). Provide a minimal default set
+    // of allowed protocols for these cases.
     $allowed_protocols = array_flip(config('system.filter')->get('protocols') ?: array('http', 'https'));
   }
 
diff --git a/core/modules/filter/lib/Drupal/filter/Tests/FilterUnitTest.php b/core/modules/filter/lib/Drupal/filter/Tests/FilterUnitTest.php
index 530ec42..b2264a4 100644
--- a/core/modules/filter/lib/Drupal/filter/Tests/FilterUnitTest.php
+++ b/core/modules/filter/lib/Drupal/filter/Tests/FilterUnitTest.php
@@ -20,7 +20,7 @@ class FilterUnitTest extends DrupalUnitTestBase {
    *
    * @var array
    */
-  public static $modules = array('system');
+  public static $modules = array('filter');
 
   public static function getInfo() {
     return array(
@@ -32,7 +32,7 @@ public static function getInfo() {
 
   protected function setUp() {
     parent::setUp();
-    $this->enableModules(array('system'));
+    config_install_default_config('module', 'system');
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/XssUnitTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/XssUnitTest.php
index 4a860c2..dff85bd 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/XssUnitTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/XssUnitTest.php
@@ -19,7 +19,7 @@ class XssUnitTest extends DrupalUnitTestBase {
    *
    * @var array
    */
-  public static $modules = array('system');
+  public static $modules = array('filter');
 
   public static function getInfo() {
     return array(
@@ -31,7 +31,7 @@ public static function getInfo() {
 
   protected function setUp() {
     parent::setUp();
-    $this->enableModules(array('system'));
+    config_install_default_config('module', 'system');
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Tests/Upgrade/SystemUpgradePathTest.php b/core/modules/system/lib/Drupal/system/Tests/Upgrade/SystemUpgradePathTest.php
index bf8321d..d8269a1 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Upgrade/SystemUpgradePathTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Upgrade/SystemUpgradePathTest.php
@@ -82,10 +82,12 @@ public function testVariableUpgrade() {
     $expected_config['system.filter'] = array(
       'protocols.0' => 'http',
       'protocols.1' => 'https',
+      'protocols.2' => 'ftp',
+      'protocols.3' => 'mailto',
     );
 
     $expected_config['filter.settings'] = array(
-      'fallback_format' => 'pagan_poetry'
+      'fallback_format' => 'plain_text'
     );
 
     foreach ($expected_config as $file => $values) {
diff --git a/core/modules/system/tests/upgrade/drupal-7.system.database.php b/core/modules/system/tests/upgrade/drupal-7.system.database.php
index 46cd25c..ca85f43 100644
--- a/core/modules/system/tests/upgrade/drupal-7.system.database.php
+++ b/core/modules/system/tests/upgrade/drupal-7.system.database.php
@@ -97,7 +97,7 @@
   ))
 ->values(array(
   'name' => 'filter_allowed_protocols',
-  'value' => 'a:2:{i:0;s:4:"http";i:1;s:5:"https";}',
+  'value' => 'a:4:{i:0;s:4:"http";i:1;s:5:"https";i:2;s:3:"ftp";i:3;s:6:"mailto";}',
 ))
 ->execute();
 
@@ -110,7 +110,7 @@
     ->condition('name', 'site_name')
     ->execute();
 db_update('variable')
-  ->fields(array('value' => 's:12:"pagan_poetry";'))
+  ->fields(array('value' => 's:10:"plain_text";'))
   ->condition('name', 'filter_fallback_format')
   ->execute();