diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index 1b746e6..755d779 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -1191,6 +1191,7 @@ public function randomString($length = 8) {
         $str .= chr(mt_rand(32, 126));
       }
     } while (isset($this->randomStrings[$str]));
+    $this->randomStrings[$str] = TRUE;
     return $str;
   }
 
@@ -1223,6 +1224,7 @@ public function randomName($length = 8) {
         $str .= chr($values[mt_rand(0, $max)]);
       }
     } while (isset($this->randomNames[$str]));
+    $this->randomNames[$str] = TRUE;
     return $str;
   }