diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index c3862bc..171df78 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -1304,16 +1304,14 @@ public function randomString($length = 8) {
     if ($length < 3) {
       return $this->getRandomGenerator()->string($length, TRUE, array($this, 'randomStringValidate'));
     }
+
     // To prevent the introduction of random test failures, ensure that the
     // returned string contains a character that needs to be escaped in HTML by
-    // splitting the length in half and injecting an ampersand into it.
-    $first = floor($length / 2);
-    $last = $length - $first - 1;
-    $parts = array();
-    foreach (array($first, $last) as $length) {
-      $parts[] = $this->getRandomGenerator()->string($length, TRUE, array($this, 'randomStringValidate'));
-    }
-    return implode('&', $parts);
+    // injecting an ampersand into it.
+    $replacement_pos = floor($length / 2);
+    // Remove 1 from the length to account for the ampersand character.
+    $string = $this->getRandomGenerator()->string($length - 1, TRUE, array($this, 'randomStringValidate'));
+    return substr_replace($string, '&', $replacement_pos, 0);
   }
 
   /**