diff --git a/core/modules/system/src/Tests/Installer/InstallerTest.php b/core/modules/system/src/Tests/Installer/InstallerTest.php
index 6d48123..0b63e04 100644
--- a/core/modules/system/src/Tests/Installer/InstallerTest.php
+++ b/core/modules/system/src/Tests/Installer/InstallerTest.php
@@ -32,4 +32,11 @@ public function testInstaller() {
     )));
   }
 
+  /**
+   * Installer step: Select language.
+   */
+  protected function setUpLanguage() {
+    $this->assertRaw('core/themes/seven/css/components/buttons.css', 'Seven\'s css is attached.');
+    parent::setUpLanguage();
+  }
 }