diff --git a/core/includes/update.inc b/core/includes/update.inc
index 510dc8c4edd1..e6fe7faded4a 100644
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -17,7 +17,7 @@ use Drupal\Core\Utility\Error;
  * Disables any extensions that are incompatible with the current core version.
  */
 function update_fix_compatibility() {
-  $extension_config = \Drupal::config('core.extension');
+  $extension_config = \Drupal::configFactory()->getEditable('core.extension');
   $save = FALSE;
   foreach (array('module', 'theme') as $type) {
     foreach ($extension_config->get($type) as $name => $weight) {
diff --git a/core/modules/system/src/Tests/Update/CompatibilityFixTest.php b/core/modules/system/src/Tests/Update/CompatibilityFixTest.php
new file mode 100644
index 000000000000..bfa752f196a5
--- /dev/null
+++ b/core/modules/system/src/Tests/Update/CompatibilityFixTest.php
@@ -0,0 +1,48 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\system\Tests\Update\CompatibilityFixTest.
+ */
+
+namespace Drupal\system\Tests\Update;
+
+use Drupal\simpletest\KernelTestBase;
+
+/**
+ * Tests that extensions that are incompatible with the current core version are disabled.
+ *
+ * @group Update
+ */
+class CompatibilityFixTest extends KernelTestBase {
+
+  protected function setUp() {
+    parent::setUp();
+    require_once \Drupal::root() . '/core/includes/update.inc';
+  }
+
+  function testFixCompatibility() {
+    $extension_config = \Drupal::configFactory()->getEditable('core.extension');
+
+    // Add an incompatible/non-existent module to the config.
+    $modules = $extension_config->get('module');
+    $modules['incompatible_module'] = 0;
+    $extension_config->set('module', $modules);
+    $modules = $extension_config->get('module');
+    $this->assertTrue(in_array('incompatible_module', array_keys($modules)), 'Added incompatible/non-existent module to the config.');
+
+    // Add an incompatible/non-existent theme to the config.
+    $themes = $extension_config->get('theme');
+    $themes['incompatible_theme'] = 0;
+    $extension_config->set('theme', $themes);
+    $themes = $extension_config->get('theme');
+    $this->assertTrue(in_array('incompatible_theme', array_keys($themes)), 'Added incompatible/non-existent theme to the config.');
+
+    // Fix compatibility.
+    update_fix_compatibility();
+    $modules = $extension_config->get('module');
+    $this->assertFalse(in_array('incompatible_module', array_keys($modules)), 'Fixed modules compatibility.');
+    $themes = $extension_config->get('theme');
+    $this->assertFalse(in_array('incompatible_theme', array_keys($themes)), 'Fixed themes compatibility.');
+  }
+}