diff -u b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
--- b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -1179,9 +1179,9 @@
   }
 
   /**
-   * Generates a unique random string containing letters and numbers.
+   * Generates a unique random string of ASCII characters of codes 32 to 126.
    *
-   * @param $length
+   * @param int $length
    *   Length of random name to generate.
    *
    * @return string
@@ -1232,7 +1232,7 @@
   /**
    * Generates a unique random string containing letters and numbers.
    *
-   * @param $length
+   * @param int $length
    *   Length of random name to generate.
    *
    * @return string