diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index a210da9..70f75d6 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -706,20 +706,20 @@ function simpletest_clean_temporary_directories($directory = NULL) {
   if (is_dir($simpletest_root)) {
     // If the $directory is not valid string or NULL then get the type of it for
     // debugging purposes in the watchdog error message below.
-    $directory = is_string($directory) && !empty($directory) ? $directory : gettype($directory);
+    $path = is_string($directory) && !empty($directory) ? $directory : gettype($directory);
     // Do not recognize any, except expected directory pattern as wrong
     // directory being passed accidentally may cause catastrophic consequences.
-    preg_match('/^(sites\/simpletest\/)(.*)/', $directory, $matches);
+    preg_match('/^(sites\/simpletest\/)(.*)/', $path, $matches);
 
     if (!empty($matches[2]) && is_dir($simpletest_root . $matches[2])) {
       $directories[] = $matches[2];
     }
-    elseif ($directory == 'NULL') {
+    elseif ($directory === NULL) {
       $directories = scandir($simpletest_root);
     }
     else {
       $logger = \Drupal::logger('Testing');
-      $logger->error('The %path is not eligible for removing in %func().', ['%path' => $directory, '%func' => __FUNCTION__]);
+      $logger->error('The %path is not eligible for removing in %func().', ['%path' => $path, '%func' => __FUNCTION__]);
     }
 
     foreach ($directories as $file) {
@@ -738,19 +738,19 @@ function simpletest_clean_temporary_directories($directory = NULL) {
     }
   }
 
-  if (!is_null($directory) && $directory != 'NULL') {
-    // We do not need anything else if this function was called from within a
-    // running test instead of using "Clean environment" button. The system
-    // messages below may cause failure on some tests.
-    // @see https://www.drupal.org/node/2745123#comment-11837053
-    return $result;
-  }
-  elseif ($count > 0) {
-    drupal_set_message(\Drupal::translation()->formatPlural($count, 'Removed 1 temporary directory.', 'Removed @count temporary directories.'));
-  }
-  else {
-    drupal_set_message(t('No temporary directories to remove.'));
+  // To prevent some tests failure do not display any messages if this function
+  // was called from within a test instead of using "Clean environment" button.
+  // @see https://www.drupal.org/node/2745123#comment-11837053
+  if ($directory === NULL) {
+    if ($count > 0) {
+      drupal_set_message(\Drupal::translation()->formatPlural($count, 'Removed 1 temporary directory.', 'Removed @count temporary directories.'));
+    }
+    else {
+      drupal_set_message(t('No temporary directories to remove.'));
+    }
   }
+
+  return $result;
 }
 
 /**