diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index 99e9d4e..fa0883d 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -134,8 +134,8 @@ public function getDefault() {
    */
   public function setDefault($theme) {
     $available_themes = $this->listInfo();
-    if (!isset($available_themes[$key])) {
-      throw new \Exception("Theme $key does not exists.");
+    if (!isset($available_themes[$theme])) {
+      throw new \Exception("Theme $theme does not exists.");
     }
     $this->configFactory
         ->get('system.theme')
diff --git a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
index 957007b..753ff5b 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
@@ -114,6 +114,7 @@ protected function setUp() {
    * @expectedExceptionMessage Theme name <em class="placeholder">thisNameIsFarTooLong0000000000000000000000000000051</em> is over the maximum allowed length of 50 characters.
    */
   public function testThemeEnableWithTooLongName() {
+    $this->themeHandler->systemList = array();
     $this->themeHandler->enable(array('thisNameIsFarTooLong0000000000000000000000000000051'));
   }
 
@@ -153,11 +154,6 @@ public function testEnableSingleTheme() {
     // Ensure that the themes_enabled hook is fired.
     $this->moduleHandler->expects($this->at(0))
       ->method('invokeAll')
-      ->with('system_theme_info')
-      ->will($this->returnValue(array()));
-
-    $this->moduleHandler->expects($this->at(1))
-      ->method('invokeAll')
       ->with('themes_enabled', array($theme_list));
 
     // Ensure the config installer will be called.
@@ -216,8 +212,15 @@ public function testEnableAndListInfo() {
     $this->assertEquals('stark', $list_info['bartik']->base_theme);
     $this->assertEquals(0, $list_info['bartik']->status);
 
+    $this->themeHandler->reset();
+
     $mock = $this->getMockTheme('seven');
-    $mock->info['status'] = 1;
+    $mock->status = 1;
+    $mock->info['stylesheets'] = array(
+        'screen' => array(
+          'style.css',
+        ),
+      );
     $this->themeHandler->systemList['seven'] = $mock;
 
     $this->themeHandler->enable(array('seven'));