diff --git a/core/tests/Drupal/Tests/Core/Test/PhpUnitCliTest.php b/core/tests/Drupal/Tests/Core/Test/PhpUnitCliTest.php
index 933c9ba8262..c31305ed978 100644
--- a/core/tests/Drupal/Tests/Core/Test/PhpUnitCliTest.php
+++ b/core/tests/Drupal/Tests/Core/Test/PhpUnitCliTest.php
@@ -36,21 +36,34 @@ class PhpUnitCliTest extends UnitTestCase {
    * Ensures that functional tests produce debug HTML output when required.
    */
   public function testFunctionalTestDebugHtmlOutput() {
-    if (getenv('BROWSERTEST_OUTPUT_DIRECTORY') === FALSE) {
+    if (!getenv('BROWSERTEST_OUTPUT_DIRECTORY')) {
       $this->markTestSkipped('This test requires the environment variable BROWSERTEST_OUTPUT_DIRECTORY to be set.');
     }
+
+    // Test with the specified output directory.
     $process = Process::fromShellCommandline('vendor/bin/phpunit --configuration core --verbose core/modules/image/tests/src/Functional/ImageDimensionsTest.php');
     $process->setWorkingDirectory($this->root)
       ->setTimeout(300)
       ->setIdleTimeout(300);
     $process->run();
-
     $this->assertEquals(0, $process->getExitCode(),
       'COMMAND: ' . $process->getCommandLine() . "\n" .
       'OUTPUT: ' . $process->getOutput() . "\n" .
       'ERROR: ' . $process->getErrorOutput() . "\n");
     $this->assertStringContainsString('HTML output was generated', $process->getOutput());
     $this->assertStringContainsString('Drupal_Tests_image_Functional_ImageDimensionsTest-1', $process->getOutput());
+
+    // Test with a wrong output directory.
+    $process = Process::fromShellCommandline('vendor/bin/phpunit --configuration core --verbose core/modules/image/tests/src/Functional/ImageDimensionsTest.php');
+    $process->setWorkingDirectory($this->root)
+      ->setTimeout(300)
+      ->setIdleTimeout(300);
+    $process->run(NULL, ['BROWSERTEST_OUTPUT_DIRECTORY' => 'can_we_assume_that_a_subdirectory_with_this_name_does_not_exist']);
+    $this->assertEquals(0, $process->getExitCode(),
+      'COMMAND: ' . $process->getCommandLine() . "\n" .
+      'OUTPUT: ' . $process->getOutput() . "\n" .
+      'ERROR: ' . $process->getErrorOutput() . "\n");
+    $this->assertStringContainsString('HTML output directory can_we_assume_that_a_subdirectory_with_this_name_does_not_exist is not a writable directory.', $process->getOutput());
   }
 
 }